Race Time Predictor: What Your 5K Time Says About Your 10K, Half Marathon, and Marathon

Race time equivalents are the finish times at other distances that match the result you already have. For example, a 25-minute 5K is the equivalent of a 52-minute 10K, a 1:55 half marathon and a 4:00 marathon.

In this guide, we cover how predictors arrive at those numbers, how to calculate your own, how much to trust each one, and how to turn your 5K into a marathon goal you can hold to the finish.

How race time predictors work

Every predictor works from a single idea: your result over one distance says something about your fitness, and that fitness can be converted into a time at another distance. The conversion scales your time up with the distance and adds a penalty, because nobody holds 5K pace for 26.2 miles.

Before you plan a race around the number a predictor gives you, you should understand two things:

1. The calculator assumes you are equally prepared for both distances. A 5K result reflects the training you did for 5K racing, and the formula treats that as though it were marathon training as well.
2. It also assumes that conditions are comparable. The same fitness produces slower times on a hilly course, on a trail, in heat and humidity, and into a headwind.

Those assumptions become more important as the gap between the two races grows.

Race time equivalents from your 5K

Treat the marathon column as a measure of marathon potential under good endurance training. A runner who races 25:00 for 5K has enough speed for roughly a four-hour marathon under this model. However, the 5K alone does not prove that the runner can hold the required marathon pace.

Popular race time calculators and race result prediction tools

Different tools can give different answers from the same 5K because they use different models or additional training data. Here are the most popular ones:

1. VDOT Running Calculator
   Jack Daniels' VDOT system estimates your current fitness from a race result and finds performances at other distances that produce a similar VDOT score. It also gives training paces.
2. McMillan Running Calculator
   This uses Greg McMillan's own race-equivalency system, built from physiology and observed race performances. McMillan also stresses that you need specific training for the predicted distance.
3. Garmin Race Predictor
   Compatible Garmin watches use estimated VO2 max and training history. If you add a race with a course attached, the prediction can also take in terrain, direction and wind speed, and the race widget shows the expected weather for the day separately.
4. Strava Performance Predictions
   Strava uses a machine-learning model based on your running history and more than 100 athlete data attributes. It requires at least 20 uploaded runs within a rolling 24-week period and predicts the major race distances separately. Predictions are available to paid subscribers and assume a flat course.

How to work out your own equivalents

Most race predictors on running sites run on one formula, published by Pete Riegel in 1977. If you would rather do the math yourself, use this formula on a phone calculator:

T₂ = T₁ × (D₂ ÷ D₁)^1.06

T2 — the time you are looking for
T1 — your known time
D2 — the distance you want to predict
D1 — the distance you ran it over
1.06 — the fatigue factor that accounts for slowing down as the race gets longer

Use miles or kilometers, as long as both distances are in the same unit, because the formula works from the ratio between them.

Tip: The power key (xʸ) appears when you turn an iPhone sideways in Calculator, or when you switch an Android calculator to scientific mode.

Say you want a marathon estimate and you have a recent 5K result. Your known time goes in as T1 with 5K as D1, and the marathon goes in as D2.

Examples:

In kilometers. Take a 24:30 5K, which is 24.5 minutes:

24.5 × (42.195 ÷ 5)^1.06 = 235 minutes, or 3:55

In miles. The same runner has 3.107 miles behind them and 26.219 miles ahead:

24.5 × (26.219 ÷ 3.107)^1.06 = 235 minutes, or 3:55

Rounding the distances to 3.1 and 26.2 shifts the answer by about 20 seconds, which is close enough for planning. The same arithmetic gives 51:05 for 10K and 1:52:42 for the half marathon.

One shortcut to remember: for the marathon, multiply your 5K time by 9.6.

How reliable are these predictions?

The further apart the two distances, the less reliable the prediction.

10K: close. A 10K is twice the distance of a 5K, and both races run on the same systems. On a flat course, most runners finish within a minute or two of the predicted time.

Half marathon: reasonable. Expect to finish within a few minutes of the prediction if you have been running regularly for several months and your long runs reach 10 miles (16 km).

Marathon: the weakest number in the table. Riegel wrote that his formula applies to efforts of 3.5 to 230 minutes. Any marathon slower than 3:50 takes longer than 230 minutes, so it falls outside that range. The formula also knows nothing about your weekly mileage or your longest run, and the marathon depends on both.

Also, there are two things that make any prediction less accurate, whatever the distance:

1. Workout times are not race times. A time trial on your own, with no other runners around you and no bib on your chest, usually comes out slower than a race effort, so predictions built on it come out slow in turn.
2. Track and short-course times flatter you. A smooth surface, a flat profile and the shortest possible line all help, so a track 5K is usually quicker than the same fitness produces on a road course with turns and a few rolling sections.

Why your marathon may come out slower than the prediction

A 5K shows your speed and aerobic fitness, but a marathon asks for much more. These are the main reasons why your actual marathon time may be slower than the number a race calculator gives you:

1. Low weekly mileage. Marathon performance depends on the amount of running you can sustain during training. Two runners can share the same 5K time but have very different marathon results if one has built much more endurance.
2. Short long runs. Regular long runs develop the endurance needed to keep the predicted pace late in the race. If your longest runs are too short, you are likely to slow down over the final miles.
3. Strong 5K speed, weaker endurance. Some runners are naturally better at shorter races or train mostly with intervals and faster sessions. Their 5K can predict a marathon time that is too quick because they have not built the endurance to hold a strong pace for 26.2 miles.
4. No marathon experience yet. Your first marathon has more unknowns. You do not yet know how well you can pace the first half, fuel for several hours, or handle the final stretch. That makes a calculator prediction less reliable.
5. Poor fueling. A 5K needs little or no race fueling. A marathon requires a plan for carbohydrate and fluids, and mistakes can add more time than any race calculator allows.
6. Starting too fast. A marathon prediction assumes you pace the race close to your target from the start. If you run the early miles too fast, you can lose far more time later than you gained at the beginning.
7. The age of your result. A 5K from two years ago predicts the fitness you had two years ago.

What 5K time do you need for a sub-4 marathon?

About 25 minutes. The table below runs the conversion backwards, from a marathon goal to the equivalent performances at shorter distances.

Read these as the shorter-distance fitness that matches the goal, and not as a pass to the finish line. A 25-minute 5K runner with 20 miles a week behind them is a long way from a four-hour marathon, while the same runner with 40 miles a week and three 20-mile long runs has a real chance.

How to set your marathon goal time from your 5K

Start with your 5K prediction, but treat it as the fastest marathon your current speed could support with enough endurance behind it. A 25:00 5K may predict about 4:00, for example, but that does not make 4:00 your race goal yet.

Use these checks before you choose your pace:

1. Use a longer race if you have one. A recent half marathon tests the endurance a marathon asks for, which a 5K cannot do. A recent 10K also adds useful information. If your 5K predicts 4:00 and your half marathon points closer to 4:15, give more weight to the half marathon.
2. Compare the prediction with your marathon training. Look at your weekly mileage and long runs. If your 5K predicts 4:00 but you have struggled to build the mileage or complete the long runs in your plan, 4:00 is probably too aggressive.
3. Test goal pace inside your long runs. Later in the training block, include sections at your planned marathon pace within some long runs. For many runners, 6 to 10 miles (10 to 16 km) total at marathon pace is enough to learn whether the effort feels controlled. You should finish these sessions feeling that you could have kept going. If the pace repeatedly becomes hard well before the end of the planned block, adjust your goal.
4. Race a half marathon before the marathon if your schedule allows it. Four to six weeks before race day gives you a much stronger fitness check than an old 5K. Put that result into the same race calculator and compare it with your original prediction.
5. Set two times instead of one. Take the faster number as your A goal and the slower one as your B goal. Run the first 20 miles (32 km) at B pace, and if the pace still feels like work you can manage, pick it up over the final 10K.

Conclusion

Race time equivalents are most useful as a way to compare your fitness across distances and set realistic targets for your next race. Use the table or a trusted calculator to get a starting estimate, then give more weight to recent results that are closer to the distance you plan to race.

As the distance gets longer, your training, endurance, pacing, and race conditions matter more, so treat every predicted time as a guide rather than a guarantee.

FAQ

How much slower is marathon pace than 5K pace?

Under the Riegel model, marathon pace comes out about 14% slower than 5K pace at every level. In minutes the gap grows as you get slower: about 58 seconds per mile (36 seconds per km) for a 22-minute 5K, and about 79 seconds per mile (49 seconds per km) for a 30-minute 5K. Recreational runners without strong marathon endurance often need a larger gap.

Which race predicts the marathon best?

A recent half marathon is usually more useful than a 5K because its demands are much closer to the marathon. A 10K also gives a stronger endurance check than a 5K. Your best estimate comes from combining a recent longer race with your weekly mileage and long-run training.

Are Garmin race predictions accurate?

Garmin predictions are useful because they draw on weeks of your own training rather than a single race result. Compatible watches use estimated VO2 max and training history, and a race with an attached course can also bring terrain, direction and wind speed into the estimate.

They remain estimates. When your watch and your recent half marathon disagree, plan your pace from the half marathon.

Can I run a marathon if I have only raced a 5K?

Yes. You do not need to race a half marathon or 10K before entering a marathon, but you should remember that marathon preparation still requires months of longer running. A half marathon during the training block can help you check your goal pace before race day.