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Predict Your Half Marathon Time from a 10K Result

Q: How old should my 10K result be for an accurate projection?

Most coaches recommend a 10K result from within the **last 6–8 weeks** for the most reliable half marathon projection. Fitness shifts by 2–5% in just 4–6 weeks of focused training or detraining. A result older than 12 weeks should be treated as a rough baseline only.

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Reviewed by: Martín Rodríguez (política editorial ) · Last reviewed: 4 jun 2026

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This calculator uses the Cameron endurance prediction formula to project your half marathon (21.1 km / 13.1 miles) finish time from a recent 10K result. Unlike Riegel's fixed-exponent model (1.06), Cameron's formula uses a dynamic exponent that scales with race time — faster runners get a slightly lower exponent, reflecting better aerobic economy; slower runners get a higher one, reflecting greater physiological fatigue over distance. The formula: T21K = T10K × (21.0975 / 10)^(1.07 + 0.0065 × ln(T10K)). For a 50-minute 10K runner, the projected half marathon is approximately 1:53 under similar training and race conditions.

Last reviewed: June 3, 2026 Verified by Martín Rodríguez Source: Riegel, P.S. (1981). Athletic Records and Human Endurance. American Scientist, 69(3), 285–290., Runner's World — Race Time Predictor (Cameron model overview) 100% private

To predict a half marathon time from a 10K result, use Cameron's formula: T21K = T10K × (21.0975 / 10) ^ (1.07 + 0.0065 × ln(T10K)). The dynamic exponent accounts for progressive fatigue. A 50-minute 10K projects to approximately 1:53 (113 min); a 45-minute 10K projects to approximately 1:42.

When to use this calculator

A recreational runner who just finished a 10K in 55 minutes wants a realistic finish-time goal before registering for a half marathon 12 weeks away.
A competitive club runner who clocked 42:00 in a 10K time trial needs to calculate their threshold pace zones for half marathon training intervals.
A first-time half marathon participant wants to estimate whether they can finish under 2:30 based on a recent park run 10K of 67 minutes.
A running coach is building pace-per-mile training targets for multiple athletes using each athlete's current 10K fitness benchmark before a spring half marathon race.

Worked Example

10K race time: 50 min
Dynamic exponent = 1.07 + 0.0065 × ln(50) = 1.07 + 0.0065 × 3.912 = 1.0954
Projected 21K = 50 × (21.0975 / 10)^1.0954 = 50 × 2.2654 ≈ 113.3 min
Result: 1 hour 53 minutes

Result: 1:53

How it works

3 min read

How Cameron's Formula Works

The Cameron endurance formula projects finish time across distances by applying a power-law relationship with a dynamic exponent:

T21K = T10K × (D21K / D10K) ^ e

Where:
  T10K  = your 10K finish time in minutes
  D21K  = 21.0975 km (official half marathon distance)
  D10K  = 10 km
  e     = 1.07 + 0.0065 × ln(T10K)  ← Cameron's dynamic exponent

Step-by-step for T10K = 50 min:
  Exponent = 1.07 + 0.0065 × ln(50)
           = 1.07 + 0.0065 × 3.912
           = 1.0954
  Ratio    = 21.0975 / 10 = 2.10975
  Factor   = 2.10975^1.0954 ≈ 2.2654
  T21K     = 50 × 2.2654 ≈ 113.3 min → 1:53:17

The exponent increases with race time because less-trained (slower) runners experience greater physiological fatigue over longer distances — their glycogen depletion, lactate accumulation, and cardiovascular drift are proportionally larger relative to their aerobic capacity.

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Reference Table — 10K to Half Marathon Projections

All times calculated with Cameron's dynamic exponent.

10K Time	Projected 21K	Avg Pace (min/km)	Avg Pace (min/mi)	Fitness Level
35:00	1:19:09	3:45/km	6:02/mi	Elite
40:00	1:30:31	4:17/km	6:54/mi	Advanced
45:00	1:41:54	4:50/km	7:46/mi	Intermediate
50:00	1:53:17	5:22/km	8:38/mi	Recreational
55:00	2:04:40	5:55/km	9:31/mi	Beginner+
60:00	2:16:03	6:27/km	10:23/mi	Beginner
70:00	2:38:51	7:32/km	12:07/mi	Walk/Run
80:00	3:01:39	8:37/km	13:51/mi	Fitness/Completion

All projections assume flat course, neutral weather (55–65°F / 13–18°C), and maximal race-day effort.

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Key Reference Numbers

Sub-2:00 half marathon requires a 10K of approximately 52:57 or faster

Sub-1:45 requires a 10K of approximately 46:30 or faster

Sub-1:30 requires a 10K of approximately 39:40 or faster

A 5-minute 10K improvement translates to roughly 11–12 minutes faster on the half marathon

The Cameron factor for most recreational runners (45–60 min 10K) ranges from 2.21 to 2.27×

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Cameron vs. Riegel: What's the Difference?

Both models use the same power-law structure, but differ in the exponent:

Riegel (1977/1981): fixed exponent of 1.06 for all runners

Cameron (1992): dynamic exponent 1.07 + 0.0065 × ln(T), which increases with race time

For a 50-minute 10K runner: Cameron's exponent is ≈ 1.095 vs. Riegel's fixed 1.06 — Cameron predicts a 3–4 minute longer finish time for recreational runners, which most coaches consider more realistic.

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Worked Examples with Training Implications

Example 1 — Recreational Runner (50:00 10K)
T21K = 50 × 2.2654 ≈ 113.3 min → 1:53:17
Target half marathon pace: 5:22/km (8:38/mi). Train with:

Long runs at 6:15–6:30/km (10:00–10:30/mi)

Tempo runs at 5:00–5:10/km (8:05–8:20/mi)

Goal race pace intervals at 5:22/km

Example 2 — Competitive Runner (42:00 10K)
Exponent = 1.07 + 0.0065 × ln(42) = 1.0704; T21K = 42 × 2.1978 ≈ 92.3 min → 1:32:18
Target pace: 4:22/km (7:02/mi). Aerobic threshold training at 4:10–4:15/km.

Example 3 — Beginner Targeting Sub-2:30 (65:00 10K)
Exponent = 1.0999; T21K = 65 × 2.2784 ≈ 148.1 min → 2:28:06
Just under the target! Maintain 7:02/km or run/walk at 6:30/km running with short walking breaks.

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Common Errors That Skew Projections

1. Using a training run instead of a race effort — Cameron's formula assumes maximal race-day effort. A casual 10K jog produces wildly inflated projections. Always use a timed, competitive result.

2. Ignoring course elevation — The formula assumes flat terrain. A hilly 10K can add 10–15 sec/km vs. flat equivalent. For courses with >150m net gain, subtract roughly 10 seconds per 100m of climbing before using the formula.

3. Hot weather bias — Performance degrades ~0.3–0.5% per 1°C above 15°C for trained runners. A 27°C (80°F) race could add 4–7 minutes to a projected 2:00 half marathon. The calculator assumes neutral ~15°C.

4. Stale 10K result — Use a result from within the last 6–8 weeks. Fitness changes 2–5% in 4–6 weeks of focused training or detraining.

5. Not accounting for training volume — The formula projects physiological potential, not training preparedness. If your long run has never exceeded 15 km, real-race performance will likely trail the projection by 5–15%.

Frequently asked questions

What 10K time do I need to run a sub-2:00 half marathon?

Using Cameron's dynamic formula, you need a 10K of approximately 52:57 (52 minutes, 57 seconds) or faster. In pace terms, that's running your 10K at about 5:18/km (8:32/mi) or quicker. Consistent tempo work at 5:00–5:10/km is the core training stimulus to build that base.

How accurate is Cameron's formula for predicting a half marathon from a 10K?

For well-trained runners with a recent, race-effort 10K, Cameron's formula is accurate to within ±3–5% under standard conditions. That represents roughly ±3–6 minutes for a 2:00 half marathon finisher. Accuracy decreases for very new runners, extreme weather, or hilly courses.

What is Cameron's dynamic exponent and why does it matter?

The Cameron exponent (1.07 + 0.0065 × ln(T10K)) is higher for slower runners because they experience greater physiological fatigue — glycogen depletion, lactate buildup, and cardiovascular drift — over longer distances relative to their aerobic capacity. A 35-min 10K runner has an exponent of ~1.083; a 60-min runner has ~1.096. This makes Cameron more conservative (realistic) than Riegel's fixed 1.06 for recreational athletes.

Should I use a race 10K result or a time trial?

Always prefer a certified race result. Race conditions — crowd energy, chip timing, competitive pacing — typically produce efforts 1–3% faster than solo time trials. If using a time trial, multiply the result by 0.97 before entering it to approximate true race-effort equivalence.

How does heat affect my projected half marathon time?

Performance degrades roughly 0.3–0.5% per 1°C above 15°C (59°F) for trained runners, and up to 1.0% per degree for recreational athletes per ACSM guidelines. A race in 27°C (80°F) could add 4–7 minutes to a projected 2:00 finish. The calculator assumes neutral temperature (~15°C / 59°F).

Can I use this if my 10K was on a hilly course?

Not without adjustment. Cameron's formula is calibrated for flat-to-rolling road courses. A hilly 10K reflects more muscular fatigue than a flat effort at equivalent fitness. If your 10K had >150 feet (45m) of net elevation gain, subtract roughly 8–12 seconds per 100 feet (30m) of climbing before running the projection.

How old should my 10K result be for an accurate projection?

Most coaches recommend a 10K result from within the last 6–8 weeks for the most reliable half marathon projection. Fitness shifts by 2–5% in just 4–6 weeks of focused training or detraining. A result older than 12 weeks should be treated as a rough baseline only.

Does the formula work the same for men and women?

Yes — Cameron's formula uses the same exponent regardless of sex, as the fatigue curve shape is physiologically similar. Performance gaps between male and female runners at equivalent fitness are already captured when you enter your actual 10K time. No sex-based adjustment is needed.

How does Cameron's formula compare to McMillan and Daniels running calculators?

All three use power-law models with similar accuracy for 10K→half marathon projections. McMillan's calculator uses a proprietary model that also accounts for athlete type (speed vs. endurance). Daniels' VDOT model is derived from aerobic capacity tables. For 10K→21K, all three methods typically agree within 2–4 minutes for recreational runners. Cameron's dynamic exponent makes it slightly more conservative than Riegel's fixed-exponent model that some calculators use.