Run a 5K last weekend and wondering what half marathon you're capable of? Just finished a 10K and need a realistic marathon goal before entering a race? This calculator takes one race result at any standard distance and projects your equivalent finish time at every other distance from 5K through marathon, using the Riegel endurance formula alongside VDOT equivalence. No sign-up, no ads, results in under a second.
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The race time predictor uses the Riegel endurance formula, developed by Peter Riegel and published in a 1981 paper in American Scientist. The formula is deceptively simple: T2 = T1 × (D2 / D1)^1.06. T1 is your known time at distance D1; T2 is the predicted time at distance D2. The exponent 1.06 captures the empirically observed fact that running performance degrades with distance at a consistent rate across most runners and most distances.
The 1.06 exponent means that doubling the race distance does not double your time — it multiplies it by approximately 2.08. A 45:00 10K runner, for example, predicts a marathon of around 3:31:50, not 3:00:00. This "fatigue factor" holds reasonably well between 5K and 50K for well-trained aerobic runners. It is less reliable at extremes: very short efforts (below 1.5 km) are more neuromuscular, and very long efforts (above 50 miles) involve fueling and sleep variables the formula does not account for.
The VDOT score shown above is Jack Daniels' shorthand for "the VO2max value that would predict your race performance at that distance." It is derived from the Daniels-Gilbert equations, which model the relationship between oxygen consumption, running velocity, and percentage of VO2max sustainable at different race durations. A VDOT of 50 from a 10K maps to roughly the same training paces as a VDOT of 50 from a half marathon — that consistency is what makes it useful.
Critically, VDOT is not your physiological VO2max. A Garmin or Polar estimate of VO2max is derived from resting heart rate and HRV algorithms; your VDOT is derived from actual race performances. The two numbers are often close but not identical. Use VDOT for setting training paces and race goals; treat your watch's VO2max estimate as a trending signal, not a precise value. When they diverge by more than 3-4 points, the race-derived VDOT is more reliable for pacing decisions.
The relationship between VDOT and training zones is straightforward: easy pace sits roughly 60-70% of VDOT velocity, threshold is around 88%, interval pace around 98-100%, and repetition pace exceeds 100%. The training pace calculator converts your VDOT into all five Daniels zones in one step.
Race time predictions are most reliable when the known result and the target distance are within a factor of about 4 of each other — 5K to half marathon, 10K to marathon. The further apart the distances, the more the prediction depends on assumptions that may not hold for your specific physiology.
The most honest use of this tool is as a sense-check, not a guarantee. If your 10K time predicts a 1:38 half marathon and you have done the long-run work, aim for 1:38. If you have not run over 14 km in your training, aim for 1:43 and build trust from there.
The most common mistake in race goal-setting is using wishful thinking rather than evidence. Runners enter a half marathon targeting 1:45 because it is a round number, not because their training data supports it. Using a verified recent race result — even a low-key parkrun — anchors the goal in performance reality.
The process that works: run a hard effort at a shorter distance three to six weeks before your target race. It does not need to be a formal race — a time trial on a flat loop is sufficient. Input that result here, read the predicted time for your target distance, and set that as your A goal. Then factor in your training consistency since the time trial. If you have been hitting 95% of scheduled sessions, trust the prediction. If you have missed two long runs, take two minutes off the target.
Get-Split's plan generator uses this same calibration logic. When you enter a goal race time, it checks whether the time is consistent with your recent activity data and VDOT. If you enter an optimistic goal, it flags the gap and offers you the option to build toward it over a longer cycle — rather than generating a plan with training paces you cannot currently hit.
A race time prediction without a training plan is just a number. The prediction becomes actionable when it drives the paces in every session between now and race day. If your 10K predicts a 1:52 half marathon, your threshold sessions should be at roughly 5:14/km, your long runs at 6:15-6:30/km, and your VO2max intervals at around 4:58/km. Those paces are not arbitrary — they are calibrated to produce the aerobic and lactate adaptations that close the gap between where you are and that 1:52 finish.
What changes with a structured plan versus running by feel: you spend the right proportion of time in each zone, you build long-run distance on a schedule that peaks and tapers before race day, and you do not accumulate fatigue in the wrong places. Most amateur runners who underperform their predictions do so because they run their easy days too hard, compressing the recovery that allows threshold and interval sessions to land at the right intensity.
Build my training plan →For distances within a factor of 3-4 of each other and for runners with balanced training across short and long efforts, predictions are typically within 2-5 minutes. The Riegel formula has been validated across hundreds of thousands of race results. Accuracy drops when the known result was run in unusual conditions (extreme heat, a hilly course) or when the runner's training is heavily skewed toward one distance — a miler predicting a marathon, for example, will get an optimistic number because their aerobic base is disproportionately underdeveloped.
The Riegel formula assumes consistent aerobic fitness across all distances. If your 5K or 10K time is strong but you have not built a marathon-specific endurance base — long runs exceeding 28–32 km, back-to-back long efforts, adequate monthly volume — the marathon prediction will be optimistic. Think of the prediction as the time you could run if the training were in place. Use it as a target to train toward over 16–20 weeks, not as a race-day expectation without the prep.
VDOT is Jack Daniels' performance-derived fitness index, calculated from race times using the Daniels-Gilbert equations. It represents the VO2max value that would explain your race performance, accounting for running economy. Your watch estimates VO2max from heart rate variability and sub-max heart rate data — a different methodology. The two values correlate but rarely match exactly. VDOT is more reliable for setting training paces because it is derived from your actual running output, not from a physiological proxy.
Yes, and it is often better. A parkrun or a solo timed effort on a flat 5 km loop removes the variables of race-day adrenaline, crowds, and drafting. The result is a cleaner signal of your actual fitness. Run it at genuine race effort — not a tempo run, not a "comfortable hard" — and the prediction will be as accurate as any formal race result. The key is maximum sustainable effort for the full distance.
Every 6–8 weeks is a reasonable cycle. Run a shorter race or time trial, update your VDOT, re-check the predictions for your target event. During a structured training block, you should see small but consistent improvements in your 5K or 10K time as your aerobic base and lactate threshold improve — and the marathon or half marathon prediction will shift accordingly. If it is not moving after 8 weeks of consistent training, that is a signal to review your training zones and session quality.
Not directly. The Riegel formula assumes road conditions and a flat-to-moderate course. Trail races with significant elevation gain (over 50 m per km) run 15–30% slower than road equivalents. Use the road prediction as a starting point, then add time for elevation. A common rule of thumb: add 8 minutes per 1,000 m of climb for reasonably fit runners, or use an adjusted "graded pace" tool specific to your expected course profile.