Running Pace Calculator

Three inputs and you’re done:

1. Distance. Enter the distance you ran or plan to run. Decimals are fine (e.g., 3.1 miles, 21.1 km). For exact race distances, use 3.1 for 5K, 6.2 for 10K, 13.1 for a half marathon, 26.2 for a marathon.
2. Distance unit. Miles or kilometers. The calculator converts internally using 1 mile = 1.60934 km.
3. Time. Break the time you took (or your goal time) into hours, minutes, and seconds. A 45-minute 5K is 0 hours, 45 minutes, 0 seconds. A 3:42:18 marathon is 3 hours, 42 minutes, 18 seconds.

The results panel shows your pace per mile and pace per kilometer in MM:SS format, your speed in both mph and kph, and projected finish times for 5K, 10K, half marathon, and marathon. The projections assume you maintain the same pace at every distance.

Common ways to use this tool: enter a workout you just finished and see what race times that pace implies; enter a goal race time and work backward to find the pace per mile you need to hit on the track; or enter a target race time at one distance to see what training pace you should aim for in shorter workouts. For pace-based training, runners typically slow down 30–90 seconds per mile from goal race pace for easy aerobic runs, and run 10–30 seconds per mile faster than goal pace for short interval work.

Running pace is the most useful number on a runner’s watch — the time it takes to cover one unit of distance, almost always one mile or one kilometer. Pace is what you actually feel when you’re running. A pace of 10:00/mile is a steady aerobic effort for most recreational runners; 6:30/mile is what an open-class miler hits at the high-school state meet; sub-5:00/mile is national-class. Same units, very different worlds.

The inverse of pace is speed — distance covered per hour. A 10:00/mile pace equals 6.0 mph. A 6:00/mile pace equals 10.0 mph. Pace is more useful for training (it makes split timing easy), while speed is sometimes more useful when comparing running to cycling or talking about treadmill settings.

This calculator does three jobs in one: it converts a finish time into pace and speed, projects how long the same pace would take you over the four most common race distances (5K, 10K, half marathon, full marathon), and reports both metric and imperial units side by side. The projections assume you can hold your current pace constant. Real race results depend on terrain, weather, fueling, and the simple physiological reality that no one holds 5K pace for a marathon — longer distances slow you down even if your fitness is the same.

For more accurate cross-distance predictions, runners often use the Riegel formula: T2 = T1 × (D2 / D1)^1.06. The 1.06 exponent (rather than 1.0) reflects the empirical observation that pace slows as distance grows. A 25-minute 5K runner does not run a 50-minute 10K; they run roughly a 51:53 10K. This calculator’s projections use a flat pace assumption, which is fine for similar distances but optimistic for jumps from a 5K pace to a marathon prediction. Treat the marathon and half-marathon projections from a short-distance run as best-case ceilings.

Disclaimer: Race-time projections are estimates only. Performance depends on training history, course profile, weather, fueling, and many other factors not captured by pace alone. Consult a coach or doctor before starting an aggressive training program, particularly if you have cardiovascular risk factors.

The math is intentionally simple — pace and speed are just unit conversions of time per distance.

Conversions and core formulas:

• Total seconds: T = h×3600 + m×60 + s
• Imperial ↔ metric: D_m = D_k ÷ 1.60934,   D_k = D_m × 1.60934
• Pace per mile: p_m = T ÷ D_m
• Pace per kilometer: p_k = T ÷ D_k
• Speed (mph): 3600 ÷ p_m
• Speed (kph): 3600 ÷ p_k
• Race-time projection: p_m × race distance in miles

For example: 10:00/mile pace = 600 seconds/mile. A 5K (3.10686 miles) at that pace would take 600 × 3.10686 = 1,864 seconds = 31:04. A marathon (26.2 mi) at the same pace would take 600 × 26.2 = 15,720 seconds = 4:22:00.

For more realistic cross-distance predictions, especially when projecting from a short race to a long one, the Riegel formula is the standard: T₂ = T₁ × (D₂/D₁)^1.06. Plug in your known time and distance, and the formula scales the prediction with a small fatigue penalty.

Example 1 — 5K training run. You finish a 5K (3.1 miles) in 31:15 (1,875 seconds).

• Pace per mile: 1,875 ÷ 3.1 = 605 seconds = 10:05/mile
• Speed: 3600 ÷ 605 = 5.95 mph
• 10K projection: 605 × 6.21371 = 1h 2m 38s
• Half marathon projection: 605 × 13.1 = 2h 12m 3s
• Marathon projection (flat-pace): 605 × 26.2 = 4h 24m 7s

The flat-pace marathon projection is optimistic. Using the Riegel formula instead: T₂ = 1,875 × (26.2/3.1)^1.06 ≈ 18,012 seconds ≈ 5h 0m 12s. The honest expected marathon time from a 31:15 5K is closer to 5 hours, not 4:24. The flat projection assumes a level of endurance most runners do not yet have at 5K fitness alone.

Example 2 — Half marathon goal pace. You finish a half marathon (13.1 miles) in 1:58:30 (7,110 seconds).

• Pace per mile: 7,110 ÷ 13.1 = 9:03/mile
• Speed: 6.63 mph
• Marathon flat-pace projection: 7,110 × 2 = 3h 57m 0s

Riegel-adjusted marathon projection: 7,110 × 2^1.06 ≈ 14,824 seconds = 4h 7m 4s. The flat doubling underestimates by about 10 minutes — a meaningful gap when you’re trying to break 4:00:00.

Example 3 — Metric-input 10K. You finish 10 km in 53:20 (3,200 seconds).

• 10 km converts to 10 ÷ 1.60934 = 6.2137 miles
• Pace per mile: 3,200 ÷ 6.2137 = 8:35/mile
• Pace per kilometer: 3,200 ÷ 10 = 5:20/km
• Speed: 6.99 mph (11.25 kph)
• 5K projection: 26m 40s (a perfectly even split)
• Marathon flat-pace projection: 3h 44m 53s

For this fitness level the flat marathon projection is closer to realistic — runners who finish a 10K under 55 minutes typically have enough aerobic capacity for the marathon to drift only 10–20 minutes off the linear projection if they train the distance properly.