跑步配速計算機

What pace measures

Pace is the inverse of speed — time per unit distance, not distance per unit time. Runners think and train in pace. A “5:00 pace” means 5 minutes per kilometer (or mile, depending on locale). It’s the natural unit for race planning because total time is a simple multiplication: pace × distance = total time.

Speed in km/h or mph is more familiar to drivers and cyclists. Both are correct; pace is the convention in running.

Pace (sec/km) = total seconds / distance (km)
Speed (km/h)  = distance (km) / time (hours)
Time          = pace × distance (with matched units)

The Riegel race-time formula

In 1977, civil engineer and competitive runner Peter Riegel published a remarkably durable formula for predicting race times across distances:

T2 = T1 × (D2 / D1) ^ 1.06

• T1 — your finish time at a known distance (D1)
• T2 — predicted finish time at the new distance (D2)
• 1.06 — the empirical exponent that encodes how much slower pace gets as distance grows

If T1 was perfectly maintainable indefinitely, the exponent would be 1.0 (linear extrapolation). The 1.06 exponent says each doubling of distance slows pace by about 4.2% (since 2^0.06 ≈ 1.042). That maps well to actual race results across populations, especially for distances between 1500m and the marathon.

A worked example

A runner finishes a 5K in 25:00 (a pace of 5:00/km). Predict the half-marathon time:

T2 = 25:00 × (21.0975 / 5.0) ^ 1.06
   = 1500 sec × (4.2195) ^ 1.06
   = 1500 × 4.524
   = 6786 sec
   = 1:53:06

Predicted half-marathon: 1 hour 53 minutes.

Now predict the marathon:

T2 = 25:00 × (42.195 / 5.0) ^ 1.06
   = 1500 × 9.443
   = 14164 sec
   = 3:56:04

Predicted marathon: 3 hours 56 minutes.

The Riegel marathon prediction implicitly assumes the runner has trained for the marathon distance. If they’re a 5K specialist who’s never run more than 10K in training, the real marathon time will likely be much slower — adding minutes for every 5K of distance their endurance hasn’t yet covered.

How to use this calculator

1. Toggle metric (km) or imperial (miles).
2. Enter your distance and your finish time (hours, minutes, seconds).
3. The calculator returns:
   • Your pace in min/km or min/mile.
   • Your speed in km/h.
   • Race-time predictions for 1500m, 5K, 10K, half marathon, and marathon via the Riegel formula.

When the Riegel prediction is wrong

The Riegel formula assumes equal training for the target distance. The two big real-world failure modes:

1. Extrapolating from short to long without long-distance training

A 5K time of 20:00 predicts a marathon of about 3:09. But if your longest training run has been 12K, your real marathon will be far slower — possibly 4:00 or more. The 1.06 exponent doesn’t model the cardiovascular and muscular demands of endurance you haven’t trained.

Rule of thumb: trust Riegel within 2× the training distance. A runner whose long runs are 25 km can trust Riegel up to about 50 km. Beyond that, marathon-specific predictions need adjustment.

2. Compressing from long to short for an untrained sprinter

A 3:30 marathoner might Riegel-predict a 21:50 5K. If they have great endurance but no top-end speed work, their real 5K could be slower (22:30 or worse). Speed needs training too.

A more conservative rule for short-distance prediction is to use an exponent of 1.07 or 1.08 for less-trained runners. Riegel himself noted in later writing that 1.06 is a population average, and individual exponents range from 1.04 (genetic talent for endurance) to 1.10 (genetic talent for speed).

Practical pace targets

Race	Recreational	Trained	Strong amateur
5K	30:00	22:00	18:00
10K	1:05:00	47:00	38:00
Half marathon	2:30:00	1:45:00	1:25:00
Marathon	5:00:00	3:45:00	3:00:00

These are recreational benchmarks. Elite times are far below — the marathon world record is under 2:01 — but those aren’t useful pace anchors for most runners.

How to pace a race

For most distances, even or negative splits beat positive splits (going out fast and dying). The cardiovascular cost of starting too hard accumulates non-linearly — every minute of overcommitting in the first 10% of a race costs you 2–3 minutes by the finish.

For a 5K:

• Run the first kilometer 5–10 seconds slower than goal pace.
• Hit goal pace for kilometers 2–4.
• Empty the tank for the last 1 km.

For a marathon:

• The first 10 km should feel annoyingly easy.
• Hit marathon pace from kilometer 10 to 35.
• The last 7 km is a battle — the goal is to slow down by no more than 5 seconds per kilometer.

When to recompute the calculator

Run a tune-up race at half your goal distance roughly 3 weeks before your goal race. Plug the result in, see what Riegel predicts, and adjust your goal pace if reality has moved the goal posts. A 5K tune-up is excellent for 10K planning; a 10K is excellent for half-marathon planning; a half-marathon is excellent for marathon planning.