How many seconds per kilometre should I slow down in the heat?

There is no universal figure. Conditions, adaptation and planned effort change the answer. This simulator only shows the calculation for a change you provide.

Why does it not calculate the percentage from temperature?

Because there is no validated coefficient that works for every runner, distance and level of acclimatisation. If you want a weather-linked reference, there is another tool with a much narrower scope.

Does it store my data?

No. The calculation is local and your entries are not sent to any server. The summary is only downloaded if you ask for it.

Hot-weather pace simulator

Heat changes the effort of the same run. Compare two of your own records or try an adjustment you choose: you will see the calculation, where it comes from and what we still do not know.

What this simulator calculates

With two of your own records it calculates the change between the hot-weather pace and the cooler-conditions pace, and applies it to a reference pace. With a hypothetical scenario it applies the percentage you type.

The resulting pace is rounded to the second and the total time is calculated from that displayed pace. The calculation runs in your browser: your entries are not sent to any server.

scenario pace = reference pace × (1 + change ÷ 100)

What it cannot tell you

The comparison does not isolate the effect of heat: fatigue, terrain or effort also change from one run to another.
The same adjustment will not necessarily repeat on another day.
Temperature and humidity are context: they do not correct the result or calculate WBGT.
It does not calculate water or sodium needs or tell you whether it is safe to run.
The input limits (paces from 1:00 to 30:00, changes from 0 to 30%, distances up to 100 km) are tool decisions, not physiological thresholds.

When it makes sense to use it

It helps you see how a change you have already observed would translate, or the arithmetic effect of an adjustment you want to explore. It does not set a race pace or tell you whether a run is appropriate at a given time.

Sources

These publications explain context and limits. They do not validate this simulator or its ability to predict an individual result.

Frequently asked questions

: There is no universal figure. Conditions, adaptation and planned effort change the answer. This simulator only shows the calculation for a change you provide.
: Because there is no validated coefficient that works for every runner, distance and level of acclimatisation. If you want a weather-linked reference, there is another tool with a much narrower scope.
: No. The calculation is local and your entries are not sent to any server. The summary is only downloaded if you ask for it.

This is a calculator. Runi is a coach that decides with you what each day calls for.

Tell Runi your goal and any aches, and your plan adjusts to you.

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Heat and marathon pace referenceAn illustrative marathon scenario from the weather, with its limits.
10K training plan for beginners (in Spanish)A general 10-week plan to get you to your first 10 kilometers.
Runi, your AI running coachA coach that decides with you what each day calls for.