Half Marathon Calculator — calculate half marathon for training and performance. Evidence-based formula and benchmarks.
This half-marathon page fixes its target at 21.0975 kilometres and predicts a finish from a known race or time trial. It places the target finish and target pace first; the distance and time fields remain the observed result that feeds the projection.
Long-distance equivalence is especially sensitive to endurance preparation. A short, fast result can be mathematically extended by the Riegel curve even when a runner has not completed the long training needed to sustain the projected pace for a half marathon.
For adjacent race-distance planning, compare this estimate with 10K Pace Calculator and Marathon Time Calculator. Treat each prediction as a planning estimate and account for course, weather, and training differences.
Choose an accurately timed effort whose intensity and course are understood, then enter it without changing the target.
The default is a 1-hour-45-minute half marathon example. It is a demonstration value, not a personal standard or recommendation.
Target time = known time × (21.0975 km ÷ known distance)^1.06. The page uses this target distance for every calculation. Target pace per kilometre is the predicted seconds divided by 21.0975; pace per mile uses the same duration divided by the half-marathon distance expressed in miles.
A 10K in 50 minutes projects to roughly 1 hour 50 minutes for a half marathon under the stated exponent. This result is a simplified equivalence, not evidence that a runner can sustain the pace without appropriate long runs, fueling practice, and pacing experience.
The finish estimate carries forward the performance level implied by the entered event, then applies the model's distance penalty. It is most useful as a first comparison across race distances; it is not a guarantee of readiness or a substitute for a recent half-marathon effort.
The two pace values convert one target time into different units. They are useful for interpreting a watch or race plan, but they omit terrain and elevation. A hilly course can require a different effort for the same average pace, and a windy route can change the result without a change in fitness.
Use a longer source race when available because it supplies more evidence about endurance. If the only source is a 5K, regard the half-marathon result as a broad estimate and test it against training and recovery over time.
Keep hydration, fueling, and conditions in a separate race plan. The formula only handles time and distance; it cannot recommend fluid, carbohydrate, or pacing decisions for an individual runner.
A half-marathon estimate can help compare training blocks when the same source-distance method is used each time. If one estimate comes from a 5K and another from a marathon, explain the difference because the two projections rely on different extrapolation ranges.
Use a target pace as a discussion point with a coach, not as an instruction to maintain one speed regardless of conditions. Hills, turns, crowding, and temperature change effort even when the distance remains standard.
Recheck the source performance after a substantial training change or a long gap away from racing. The calculator has no date input and cannot know that an old result no longer reflects present fitness.
A half-marathon pacing plan may include planned changes around hills or aid stations, while this result reports only an average. Do not expect each kilometre split to equal the average pace, particularly when course conditions are uneven. Recheck the source performance after a substantial training change or a long gap away from racing; this page has no date field and cannot determine whether an old result still describes current fitness.
The target is 13.109 miles when converted from 21.0975 kilometres. Minor differences in displayed pace can appear because the application rounds the duration to whole seconds before showing the unit conversion.
A half-marathon is just over four times the distance of a 5K. The exponent therefore has a meaningful effect when projecting from a short race, making the result more sensitive to whether the runner's endurance resembles the assumed curve.
Do not confuse the target distance with the source distance. Entering the half-marathon distance as the known result means the page projects from a half marathon; it does not force the source to be a shorter race.
Do not use an easy long-run pace as though it were a maximal race result. The model assumes the source performance reflects a comparable effort, and an easy training run has a different purpose.
Do not interpret a single optimistic projection as proof that a runner is ready for the full distance. The time equation does not check completed long runs, recovery, fueling tolerance, or injury history.
This is a general sports estimate, not medical screening or a personalized endurance plan. Increase training gradually and seek qualified advice where health, injury, or return-to-running concerns apply.