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🏃 Fitness & Health · Rex's Toolbox

Marathon Time Predictor

Project a marathon or half finish time from a recent race result.

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Rex says

Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.

Try a scenario

Click to load — tweak from there.

Inputs

Result

Projected marathon

230

Projected half marathon

110

Projected 10K

50

Projected 5K

24

Marathon pace (min/mi)

8.79

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How to use this

  1. 1Enter recent race distance (mi).
  2. 2Enter finish time (min).
  3. 3Read your projected marathon on the right — it updates as you type.
  4. 4Hit Share to keep the scenario or send it to someone.

About this calculator

Riegel's formula projects longer-distance performance from a recent shorter race. Enter what you ran and how fast, and see the projected marathon, half, 10K, and 5K equivalents.

FormulaT2 = T1 × (D2 ÷ D1)^1.06 (Riegel).

Worked example

Using the values the calculator loads with:

Inputs

  • Recent race distance: 6.2 mi
  • Finish time: 50 min

Results

  • Projected marathon: 230
  • Projected half marathon: 110
  • Projected 10K: 50
  • Projected 5K: 24
  • Marathon pace (min/mi): 8.79

What each field means

Inputs

Recent race distance (mi)
The recent race distance used in the calculation, measured in mi. Starts at 6.2 mi so you have a working example on load.
Finish time (min)
The finish time used in the calculation, measured in min. Starts at 50 min so you have a working example on load.

Results

Projected marathon
Returned as a length of time and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Projected half marathon
Returned as a length of time. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Projected 10K
Returned as a length of time. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Projected 5K
Returned as a length of time. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
Marathon pace (min/mi)
Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.

FAQ

How accurate is this?

Good within a few percent for trained runners projecting up to about double the input distance. It flatters undertrained runners projecting from a 5K to a marathon.

Why 1.06?

It's the empirical fatigue exponent Pete Riegel derived from race data. Well-trained endurance athletes sometimes sit closer to 1.05.

Accuracy and limitations

  • Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
  • Results do not account for medical conditions, medication, or pregnancy.
  • Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.

Related tools

Running Pace Calculator

Pace per mile and km, plus finish times for common races.

Calories Burned Calculator

Energy burned using MET values for common activities.

Cite this calculator

Writing about this topic? Grab a citation — every link helps keep these tools free.

APA
RevenueLab. (2026). Marathon Finish Time Predictor. Retrieved from https://www.revenuelab.fyi/toolbox/marathon-finish-time
HTML
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/marathon-finish-time" target="_blank" rel="noopener">Marathon Finish Time Predictor — RevenueLab</a> (2026).</p>
Markdown
Source: [Marathon Finish Time Predictor — RevenueLab](https://www.revenuelab.fyi/toolbox/marathon-finish-time) (2026).
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