Marathon Run Time Calculator

Running a marathon is a huge goal, and knowing what time to aim for makes training and pacing far easier. Set your target too fast and you risk hitting the wall; too slow and you may leave time on the course. A realistic prediction based on your current fitness is the best starting point.

Our Marathon Run Time Calculator predicts your finish time from a recent race, and shows your target pace and even splits. This guide explains how the prediction works, its limits and how to use it on race day.

How marathon time prediction works

The calculator uses Riegel's formula, a well-known equation from the 1970s that predicts performance over one distance from another:

Predicted time = known time × (new distance ÷ known distance) to the power 1.06

The exponent of 1.06 reflects the fact that you slow down as distances get longer. Riegel's formula is widely used by runners and coaches because it's simple and works reasonably well for trained runners.

How to use the calculator (Marathon Run Time Calculator)

1. Choose a recent race or time trial: 5K, 10K, half marathon or another distance.
2. Enter your time in hours, minutes and seconds.

The calculator predicts your marathon time, your average pace per kilometre and per mile, and your predicted 10K and half marathon times. It also gives even-pace splits at every 5K, halfway and the finish.

A worked example

If you recently ran a half marathon in 1:55:00, the predicted marathon time is about 3:59:46, an average pace of around 5:41 per km or 9:09 per mile. That suggests a sub-4-hour marathon is realistic with good training.

A 25-minute 5K gives almost exactly the same prediction, because both times reflect similar fitness. Try your own results in the Marathon Run Time Calculator.

Which race gives the best prediction?

The closer the distance to a marathon, the more accurate the prediction. A recent half marathon is the best guide. A 10K is good, while a 5K is useful but more likely to overestimate your marathon pace, because it relies more on speed than endurance.

Why you might run slower than predicted

The prediction is most accurate if you've:

• built up your weekly mileage over several months
• done regular long runs of 28 to 35km
• practised fuelling and hydration during long runs
• tapered before race day

Weather, hills, crowds and how you pace the first half all make a difference too. A hot day can add many minutes.

Pacing on race day

Even pacing, or running the second half slightly faster (a negative split), is usually the most efficient strategy. Starting too fast is the most common mistake: the adrenaline makes early miles feel easy, but going out even 10 seconds per km too quickly can lead to a big slowdown later.

Use the splits from the calculator to plan your race. Many runners write key split times on their arm or set a pace alert on their watch.

Training to improve your time

• Easy running builds your aerobic base. Most of your weekly running should feel comfortable.
• Long runs build endurance and teach your body to use fuel efficiently.
• Threshold sessions raise the pace you can sustain. Our Norwegian singles calculator gives sub-threshold paces.
• Marathon-pace runs help your body and mind get used to target pace.
• Rest and recovery are when your body adapts, so don't skip easy days.

Fuel and hydration

Your body can only store enough carbohydrate for about 90 to 120 minutes of hard running, so most marathon runners take on carbohydrate during the race, often energy gels or drinks every 30 to 45 minutes. Practise your fuelling plan in training. Drink to thirst and avoid drinking excessive amounts of water, which can be dangerous. Our water intake calculator explains daily needs.

Starting from scratch

If you're new to running, the NHS Couch to 5K programme is a great way to start. Build up gradually, and give yourself plenty of time before attempting a marathon. Many people spend a year or more progressing from 5K to half marathon to marathon.

Choosing your first marathon

For a first marathon, many runners choose a flat course with good crowd support and plenty of time to train, usually at least 16 to 20 weeks from a solid base. Look at the course profile, the typical weather for the date and the time limit. Some popular UK marathons use a ballot for places, while others offer charity places or general entry.

Using a tune-up race

A half marathon about four to six weeks before your marathon is an excellent way to test your fitness and update your prediction. Enter that result in the calculator to fine-tune your marathon target pace.