⏵ Training guide · Free

Heat Training and Acclimatization for Ultramarathons

Heat training for an ultra just means you put your body in the heat on purpose for 7 to 14 days before the race so it adapts, and most of the payoff lands in the first 4 to 6 days. You can run easy in the heat, or if you live somewhere cold you can use a post-run sauna or hot baths. Both work. What you get is more blood plasma, a lower heart rate, earlier sweating, and a real edge on race day, plus you walk in knowing how to fuel, cool, and pace so the heat does not end your race.

What heat acclimation actually does

Heat is one of those things that wrecks more races than it should, and most of those races get lost by people who never trained for it. The good news is your body adapts fast, and it adapts in a way you can count on. Hit it with heat over and over and a whole stack of changes kicks in, and the biggest one (a bigger blood volume) helps you in cool weather too. Here is what adapts, and roughly when.

⏵ The adaptations

What changes, and when it kicks in

Adaptation	Timing	Why it helps
Plasma (blood) volume expands	Days 3 to 6	Up 4.5 to 13%, and this is the big one. More blood to cool you and feed muscle at the same time.
Heart rate drops at a given pace	Days 4 to 7	Less strain on the heart, so you hold the same effort with a calmer engine.
You start sweating sooner and more	Days 5 to 10	Sweat rate can climb 50 to 100% above baseline, so you dump heat faster.
Sweat gets less salty	Days 7 to 14	Your body grabs the sodium back before it leaves, so you hold onto electrolytes (but this one varies a lot person to person).
Core temperature runs lower	Days 7 to 14	You hit any given core temp later, which buys you a bigger margin before things go sideways.

Plasma volume going up 4.5 to 13% and sweat rate going up 50 to 100% over baseline are the two numbers the research keeps coming back to. And that bigger blood volume is the reason heat work quietly makes you faster in cool weather too.

How many days do you need?

Plan on 7 to 14 days of near-daily heat for the full adaptation, but here is the thing: it is front-loaded. Roughly 75 percent of the changes show up in the first 4 to 6 days. That is good news if you are short on time before a hot race.

The block, the maintenance, and the cutoff

A plan that works well is a 10-day block of back-to-back (or close to back-to-back) heat sessions that wraps up about 7 to 10 days before the race. Once you have the block in, you do not have to grind it daily. The adaptations stick around, and one session every three days or so keeps them. Stop the dedicated heat sessions about 7 to 10 days out so you are not stacking extra stress into race week, then let the taper do its job.

But if you stop cold, the gains fade. Figure on losing roughly 2.5 percent of them per day once you go past the first 48 hours with no heat, so a full two-week gap can wipe out a real chunk of what you built. That is exactly why the maintenance sessions and a short re-acclimation close to the race matter. And if you are traveling to a hot venue, get there a few days early and you can re-acclimate further in as little as 4 days.

Four ways to get the heat in

You have a few options here, ranked from most race-specific down to easiest to actually pull off. Training in real heat is the gold standard. But post-run sauna and hot baths get you almost all the way there, and they are how cold-climate runners get ready for hot races. Whatever you pick, keep the running easy. Heat is extra load piled on top of your training.

⏵ Acclimation methods

Pick the one you can actually do consistently

Method	How to do it	Notes
Train in the heat	Run easy at ~60 to 65% effort in the heat, 50 to 100 min, once daily for 7 to 10 days.	The closest thing to the real race. It is also the hardest on recovery, so keep it easy and watch your load.
Post-run sauna (dry)	~30 min at ~175 to 195F right after a run, building from 15 to 20 min up to 30+ min.	Best-studied passive method. Do not hydrate inside, delay rehydrating ~30 min after.
Hot bath immersion	Soak to the chest in ~104F water for ~40 min right after an easy run.	No gym needed. Cheapest at-home option, nearly as effective as the sauna.
Overdress / layer up	Add layers on easy runs to trap heat and drive core temp up on purpose.	Last resort if you have no sauna or tub. Crude but real, used by cold-climate runners.

For the sauna, the version with the most research behind it is about 30 minutes after a run, built up over a bunch of sessions, with no drinking inside and rehydration held off about 30 minutes after so you get the most out of the blood-volume response. Always sauna with someone around, and step out if you start to feel faint.

Fluid and sodium when it is hot

Heat changes the math on hydration. You sweat more and you lose more sodium, and getting either one wrong costs you. Too little and you cook, too much fluid and you flirt with hyponatremia. The numbers are personal, so honestly the real answer is to measure yourself.

Drink to thirst, salt to your sweat

Heat can shove your sweat rate 50 to 100 percent above your cool-weather baseline, and that baseline is already around 0.7 to 1 liter per hour. So for most runners that means leaning toward the high end of about 16 to 28 ounces of fluid per hour in the heat, drunk to thirst and not forced down. Forcing fluids past thirst is how people end up with exercise-associated hyponatremia, which is a dangerous watering-down of your blood sodium, so let thirst lead instead of a rigid schedule.

Sodium losses climb too. A lot of ultrarunners do fine on roughly 300 to 600 mg of sodium per hour in cool weather, but heavy or salty sweaters out in real heat can need 700 to 1000 mg or more per hour. Here is the catch: acclimation itself makes your sweat less salty over time, and how salty your sweat is in the first place swings wildly from person to person, so any generic salt number is really just a guess. Weigh yourself before and after a long hot run to learn your sweat rate (each pound you drop is about 16 ounces of fluid), and dial in your hourly sodium in training so race day is something you have already rehearsed.

Want the numbers in more detail? Dig into our guides on how much sodium per hour for ultra running, how many carbs per hour, and how to build an ultramarathon fueling plan. If heat is wrecking your stomach, see how to avoid stomach problems.

How much heat slows your pace

In the heat the same effort just costs you more, because blood gets pulled out to your skin to cool you off, so your heart rate and how hard it feels both climb at any given pace. The honest move is to run by effort, not by some cool-weather goal pace. Here is the rough hit by conditions.

⏵ The pace penalty

Expect to run slower (and that is fine)

Conditions	Roughly how much slower
Cool and dry (under ~55F)	Baseline, no penalty
Warm, ~70F, low humidity	~2 to 4% slower
Hot, ~80F, moderate humidity	~4.5 to 6% slower
Hot and humid, ~85F+, dew point 70+	~12 to 15% slower

Rough rule of thumb: faster runners lose about 1 second per mile per 1C (1.8F) above ~59F, and slower runners 4 to 4.5 seconds per mile per 1C. And humidity (dew point) matters just as much as the temperature. Pace the climbs and the heat by effort with our pace-an-ultra-by-effort guide.

Avoiding heat illness on race day

Heat illness runs on a spectrum, and the line you do not want to cross is heatstroke. Prevention is three things together: being acclimated, pacing honest, and cooling nonstop. Know the signs, and when in doubt, slow down and cool off.

Cool early, cool often

Do not wait until you feel hot. From the very first aid station in the heat, make cooling part of your routine: ice in a bandana around your neck, ice in a hat, ice down the front and back of your shirt, ice in your hydration vest, water dumped over your head, and cold drinks or an ice slurry. This stuff works, inside and out, and it is meant to go alongside acclimation, not instead of it. Start conservative too, because going out too hard in the heat is the single fastest way to get yourself in trouble.

Drink to thirst, not on some forced schedule, so you stay clear of both dehydration and hyponatremia. Get in the shade on long exposed climbs, and treat a cold creek crossing or a sponge station as a chance to cool down, not just a chore.

Heat exhaustion vs heatstroke: know the line

Heat exhaustion shows up as heavy sweating, dizziness, headache, nausea, weakness, and cramping. That is a stop-and-cool signal: get in the shade, douse and ice yourself, sip some fluid, and do not push on until you come back around. Heatstroke is the real emergency. It is a core temperature crossing roughly 104F along with central-nervous-system stuff: confusion, slurred speech, stumbling, losing coordination, or going down. The fix is immediate, aggressive cooling (cold-water immersion is best) plus emergency medical help, and minutes matter.

If you or somebody near you starts acting confused or uncoordinated in the heat, treat it as heatstroke until proven otherwise: stop, cool hard, and get help. No finish is worth it. And dodging a heat blow-up is one of the most common ways to keep your race alive when the conditions turn on you.

Prepping for a hot race (Western States, Badwater, hot trail 50Ks)

The big hot races pay off the people who treat heat as something you can train, and who prep for it on purpose. The plan is the same whether your race is a canyon-heat 100 or a hot Southern California trail 50K: build your fitness, drop a heat block into the taper window, and rehearse the cooling and the fueling so nothing is new on the day.

A pro-style heat plan

A common way to do this for a race like Western States is a roughly 10-day passive heat block (post-run sauna or hot baths) dropped into the final 2 to 3 weeks, usually after a lighter exposure earlier in the build. Throw in two to three maintenance sessions a week between blocks, then stop the dedicated heat about 7 to 10 days out so you show up rested. Practice your whole cooling kit (ice bandana, vest ice, dousing) on hot training runs, and lock in your hot-weather fluid and sodium so the race-day numbers are something you rehearsed, not guessed.

For the truly extreme stuff like Badwater, the same ideas hold, but you lean even harder on nonstop cooling, a strong crew, conservative early pacing, and throwing out any cool-weather goal time. Wherever you race, go in expecting a slower clock and a smarter, cooler race. And if you are racing a hot trail event in Southern California, our course guide for the canyon-and-chaparral heat of the Cuyamaca 100K walks you through the specific terrain.

Race-specific: the Cuyamaca 100K course guide. Mountain heat often comes with thin air too, so pair this with altitude training for a mountain ultra. And get your head right with mental strategies for the low patches.

⏵ Free calculators for a hot race

Take the heat ranges in this guide and turn them into your own numbers with our free, no-signup tools.

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⏵ Tie heat prep to your actual plan

Heat work is extra stress, so it has to fit inside your real training load, not just float next to it. Summit Line reads your actual training, builds a plan dialed to YOUR fitness, generates an hour-by-hour fueling and sodium schedule you can ramp up for the heat, and projects a course-aware finish so you go in with a real, heat-adjusted target instead of a cool-weather fantasy.

Build my plan free →See it on a real plan

Keep reading

Heat is just one piece of getting an ultra right. Here is where to go next.

Heat training FAQ

How do I acclimatize to heat for a race?

Put your body in the heat over and over so it adapts before race day. The most race-specific way is to run easy in the heat at about 60 to 65 percent effort for 50 to 100 minutes, once a day. If you cannot train in real heat, passive methods get you almost all the way there: sit in a dry sauna for about 30 minutes right after a run, or soak to the chest in roughly 104F water for about 40 minutes post-run. Do it on consecutive days so the stimulus stacks, keep the running easy because heat is extra stress, and rehydrate after the passive sessions rather than during them. The goal is to nudge your core temperature up to roughly 101 to 102F and hold it there, which is the signal that triggers the adaptations.

How many days of heat acclimation do I need?

Plan on 7 to 14 days of near-daily heat for the full adaptation, but you get most of the benefit early: roughly 75 percent of the changes happen in the first 4 to 6 days. Plasma volume starts expanding within 3 to 6 days, heart rate and core temperature control sharpen over the first week, and the shift from saltier sweat to less salty keeps going out to about two weeks. A plan that works is a 10-day block that finishes 7 to 10 days before the race, then maintenance. And if you are traveling to a hot race and short on time, even 4 to 6 focused days will move the needle.

Does sauna heat training work?

Yes. The post-run sauna is one of the most-studied passive ways to acclimate. In a landmark study, runners who did about 30 minutes of post-run sauna roughly 3 to 4 times a week for three weeks expanded their plasma volume by over 7 percent and pushed their time to exhaustion up about 32 percent, with a small bump in 5K time. The version that works: get in right after a run while your core is already warm, stay 15 to 30 minutes (build up over sessions toward 30 plus), do not drink inside, hold off on rehydrating about 30 minutes afterward to get the most out of the blood-volume signal, and stop the sauna sessions 7 to 10 days before the race. A hot bath at about 104F for 40 minutes is a near-equal at-home swap.

How much more fluid and sodium do I need in heat?

More of both, but how much is personal. Heat can shove your sweat rate 50 to 100 percent above your cool-weather baseline (which is already around 0.7 to 1 liter per hour), so your fluid needs climb right along with it, usually toward the high end of about 16 to 28 ounces per hour, drunk to thirst and not forced. Sodium losses go up too: a lot of ultrarunners do well on roughly 300 to 600 mg of sodium per hour in cool conditions, and heavy or salty sweaters out in real heat can need 700 to 1000 mg or more per hour. Here is the catch: acclimation actually makes your sweat less salty over time, and how salty your sweat is in the first place swings wildly from person to person. The only way to really know is to weigh yourself before and after long hot runs to learn your sweat rate, and to dial in your hourly sodium in training. See our sodium-per-hour guide and the fueling calculator below.

How much does heat slow my pace?

Enough that pacing by the clock in heat is a trap. As a rough guide, faster runners lose about 1 second per mile for every 1C (1.8F) above roughly 59F, while slower runners can lose 4 to 4.5 seconds per mile per 1C. In practice a warm 70F day might cost you 2 to 4 percent, a hot 80F day around 4.5 to 6 percent, and a truly hot, humid day with a high dew point can slow you 12 to 15 percent. Humidity matters as much as temperature because it kills your evaporative cooling. So the takeaway is simple: in heat, run by effort and heart rate, not by a goal pace, and expect a slower finish that is not a sign of lost fitness.

How do I avoid heat illness in a race?

Prevention is acclimation plus active cooling plus honest pacing. Acclimate ahead of time so your body can handle the load, then start conservative, because going out too hard in heat is the fastest route to trouble. Cool hard and nonstop: ice in a bandana or hat, ice in your hydration vest, ice down the front and back of your shirt, water dumped over you at aid stations, and cold drinks or an ice slurry. Drink to thirst instead of overdrinking, because forcing fluids can cause exercise-associated hyponatremia (dangerously low blood sodium), which is its own emergency. Learn the warning signs: heat exhaustion brings heavy sweating, dizziness, nausea, and cramping, while heatstroke is the real emergency, marked by confusion, stumbling, or loss of coordination, and it demands immediate aggressive cooling (cold-water immersion is the gold standard) and medical help. When in doubt, slow down, get in the shade, and cool off.

How do I prep for a hot race like Western States or Badwater?

Treat heat as something you can train, and start early. A common pro move for a race like Western States is a roughly 10-day heat block dropped into the final 2 to 3 weeks (usually post-run sauna or hot baths), built on top of general fitness, then 7 to 10 days of taper with the sauna stopped about a week out so you are not adding stress into race week. Practice your full cooling kit in training (ice bandana, vest ice, dousing) and rehearse your hot-weather fueling and sodium so nothing is new on race day. For the truly extreme stuff like Badwater, the same ideas apply, but you lean even harder on nonstop cooling, crew support, conservative early pacing, and not chasing a cool-weather goal time. And if you are racing a hot Southern California trail event, our race guides cover the specific course.

Can I acclimate if I live somewhere cold?

Yes, and this is exactly what passive heat training is for. You do not need a hot climate to get hot. The easiest options are a post-run dry sauna (about 30 minutes) or a hot bath (about 104F for 40 minutes), done on consecutive days, both of which drive your core temperature up and trigger the same plasma-volume and cardiovascular changes as training in the heat. If you have neither, overdressing on easy runs to trap heat is a crude but real fallback that cold-climate athletes use. Plan your block so it wraps up shortly before you travel to the race, and if you get there a few days early you can re-acclimate further in as little as 4 days.

This guide is for general training and planning, and it reflects expert-consensus ranges. It is not a stand-in for personalized coaching or medical advice. Heat acclimation, sweat rate, and sodium needs swing a lot from runner to runner, so treat the numbers here as starting ranges and adjust to your own body, your race, and the conditions. Heat training is added physical stress: never sauna alone, step out if you feel faint, and if you have any heart, blood-pressure, or other health condition, or you are pregnant, check with a clinician before you start heat work. Exertional heatstroke is a medical emergency, so when in doubt, stop, cool hard, and seek help.