Pace Calculator
Convert between pace, speed, and finish time for any distance
Your pace
Finish times at this pace
The Pace Calculator turns any two of distance, time, and pace into the third, then shows what your current pace means for the four standard race distances. It supports kilometers and miles, and every result is computed in the browser. The math is the same simple relationship runners use on every watch: pace is time divided by distance.
Three Modes and the Formulas Behind Them
All three modes share one formula. Pace in seconds per unit is total time in seconds divided by distance in the same unit:
pace = totalSeconds / distance
finishTime = paceSeconds * distance
speed = 3600 / paceSeconds
- Distance + time to pace solves the first equation. A 10 km run in 50:00 gives 3000 / 10 = 300 seconds per km, or 5:00/km.
- Pace + distance to time solves the second. The same 5:00/km over a half marathon is 300 x 21.0975 = 6,329 seconds, or 1:45:29.
- Target time to pace is the first equation again, labeled as the required pace for a goal finish.
Speed is always the reciprocal of pace scaled to an hour, so a 5:00/km pace is 3600 / 300 = 12.0 km/h.
How to Use
- Pick a mode: Distance + time, Pace + distance, or Target time. The visible inputs change with the mode.
- Choose kilometers or miles. English defaults to miles; the distance field label and the per-unit suffix update immediately.
- Enter the numbers. Time is split into hours, minutes, and seconds fields, so a 50 minute 30 second run is 50 and 30, not 50.5.
- Read the headline result and the race table underneath. The table recalculates on every keystroke and when you toggle units.
Worked Examples
Example 1: analysing a completed 10K. Enter 10 km and 50:00. The headline is 5:00/km with a speed of 12.0 km/h. The race table then predicts:
| Race | Distance | Finish at 5:00/km |
|---|---|---|
| 5K | 5 km | 25:00 |
| 10K | 10 km | 50:00 |
| Half marathon | 21.0975 km | 1:45:29 |
| Marathon | 42.195 km | 3:30:59 |
Example 2: converting a mile pace. With miles selected, an 8:00/mile pace is 480 seconds per mile. Dividing by 1.609344 gives about 4:58/km, and the speed line reads 7.5 mph. The race predictions are identical to the 5:00/km example, within rounding.
Example 3: planning a sub-two-hour half. Choose Target time, keep kilometers, enter 21.1 and a time of 1:59:59. The required pace is about 5:41/km, or 9:09/mi.
Pace, Speed, and Unit Conversion
Pace and speed describe the same effort from opposite directions, and mixing them is a common source of confusion. A treadmill that shows 10 km/h is asking for a 6:00/km pace; a watch that shows 5:30/km corresponds to about 10.9 km/h. The conversion constant is exact: one mile is 1.609344 kilometers, so a per-mile pace divided by 1.609344 is the per-kilometer pace, and a per-kilometer pace multiplied by 1.609344 is the per-mile pace. The race distances are stored in kilometers, which is why the table stays consistent when you flip the unit switch.
Choosing Kilometers or Miles
The unit toggle is more than cosmetic. Races are usually measured in kilometers outside the United States, while American runners often think in miles, and the calculator stores its race distances in kilometers so both views agree. The default follows the page language: English starts in miles, while Chinese and German start in kilometers.
The important habit is to decide on one unit before typing numbers. A distance of 5 means five kilometers with the km toggle active and five miles with the mi toggle active, which is nearly 8.05 km. If a result looks far too fast or too slow, the unit is the first thing to check. The race table itself is immune to the confusion because it converts your pace to a per-kilometer value internally, so predicted finish times for the 5K, 10K, half, and marathon are the same whichever unit you display. Only the pace and speed lines change format.
Race Predictions and Their Limits
The table is deliberately simple: it holds one pace constant across every distance. That is useful for goal-setting but optimistic over long races. Most runners slow by roughly 5 to 15 seconds per kilometer when moving from a 10K to a marathon, and hot weather, hills, and poor fueling can add much more. The half marathon and marathon rows should be read as a pace you would need to sustain, not a time you are guaranteed to hit. The 5K and 10K rows are closer to realistic for most people because fatigue matters less over short distances.
Common Mistakes
- Entering decimal minutes. The time fields are hours, minutes, and seconds. Type 50 minutes 30 seconds, not 50.5 in the minutes box.
- Forgetting which unit is active. If the unit is km, a distance of 10 means 10 km, not 10 miles. Check the suffix before trusting the pace.
- Reading a treadmill speed as a pace. 12 km/h is 5:00/km; it is not 12 minutes per kilometer.
- Treating the marathon prediction as a race plan. Add time for the distance rather than assuming even splits.
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Frequently Asked Questions
How does the calculator convert a per-mile pace to a per-kilometer pace?
It divides the per-mile pace in seconds by 1.609344, the exact number of kilometers in a statute mile. An 8:00/mile pace is 480 seconds, and 480 divided by 1.609344 is about 298.3 seconds, or 4:58/km. The unit toggle performs this conversion automatically.
What is the difference between the Distance + time and Target time modes?
Both take a distance and a total time and divide one by the other to get a pace. Target time is simply the same calculation with the result labeled required pace, so you can approach race planning by typing the distance and your goal finish. The underlying math is identical.
Why is the marathon prediction faster than I could actually run?
The race table multiplies your current pace by each official distance and assumes you hold that pace for the entire race. Real races slow down with distance because of fatigue, fueling, and hills, so the marathon row is a best-case estimate rather than a forecast. Treat it as a target, not a guarantee.
Does the race table use my selected unit?
Yes. The four race distances are fixed in kilometers (5, 10, 21.0975, and 42.195), so if you have selected miles the calculator first converts your per-mile pace to a per-kilometer pace and then computes each row. Switching units changes the displayed pace but predicts the same finish times.
Why are the half marathon and marathon distances 21.0975 and 42.195 km?
Those are the standard road-racing distances: 21.0975 km is 13.1094 miles and 42.195 km is 26.2188 miles. The tool stores them in kilometers because that is the more precise common denominator, then converts per-mile paces when needed.
What pace do I need to break two hours in a half marathon?
Two hours is 7,200 seconds, but a 1:59:59 finish is 7,199 seconds over 21.0975 km, which works out to about 5:41 per kilometer or 9:09 per mile. Type 21.1 km and 1 hour 59 minutes 59 seconds in Target time mode to see the same figure.
Reviewed by the HandyTools Hub team · Last updated