What this page does · How the numbers are calculated · +6 more · 8 questions
What this page does
A pace difference calculator answers the question a target time leaves open: what does this much per kilometre actually buy me at the distance I race. A pace difference is a rate — seconds per kilometre, seconds per mile. A race time difference is an amount — minutes and seconds over a measured distance. The two are related by one multiplication, and this page does it in both directions, for 5K, 10K, the half marathon and the marathon.
No other page on this site converts a difference. The race pace calculator takes two of pace, time and distance for one race and returns the third; it has no notion of a difference between two paces. The cutoff pace calculator works the other way round, from a time limit to the slowest pace that still makes each checkpoint. The predictor on the home page moves a recent result along a distance curve, which is a claim about what you could run; this page makes no such claim. It takes the difference you name and prices it.
Everything runs in your browser. Nothing you type is sent to a server, there is no account and no upload, and the numbers never leave the page. The arithmetic is one multiplication, one unit definition and one division, all written up below.
How the numbers are calculated
Let d be the pace difference in seconds per kilometre and D the race distance in kilometres. If the difference is held for the whole race, the time it is worth is:
ΔT = d × D
That is the entire model. The four distances this page uses are the measured race lengths the rest of the site uses: 5 km, 10 km, 21.0975 km for the half marathon and 42.195 km for the marathon. They are the distances World Athletics ratifies, not rounding choices.
The per-mile column is the same difference expressed in the other unit. One mile is 1609.344 metres exactly, fixed by the 1959 international yard and pound agreement, so a difference measured over a mile is 1.609344 times the same difference measured over a kilometre:
dmi = dkm × 1.609344 dkm = dmi ÷ 1.609344
The percentage figure is a different statement about the same difference. It divides the difference by a base pace, so it needs one:
share = d ÷ pbase × 100%
The reverse direction is the same multiplication solved for d: a time difference ΔT over a distance D implies d = ΔT ÷ D seconds per kilometre, and that divided by 1.609344 in seconds per mile. That is what the four-box block on the page prints.
- d — the pace difference, in seconds per kilometre or per mile, as you typed it. 0.1 to 600.
- D — the race distance in kilometres. 5, 10, 21.0975, 42.195.
- ΔT — the time difference, in seconds, printed as a clock once it passes a minute.
- pbase — the pace the percentage is a share of. Your number, not a default the page invented.
- 1 mile = 1609.344 m exactly — the 1959 international yard and pound agreement. A definition, not a measurement.
A worked example: five seconds per kilometre
Take d = 5 s/km and a base pace of 5:00 per kilometre, which is 300 seconds. Over 5 km the difference is 5 × 5 = 25 s. Over 10 km it is 5 × 10 = 50 s. Over the half it is 5 × 21.0975 = 105.4875 s, printed 1:45. Over the marathon it is 5 × 42.195 = 210.975 s, printed 3:31. Per mile the same difference is 5 × 1.609344 = 8.05 s. As a share of the 300-second base pace it is 5 ÷ 300 = 1.67%. The new pace, if the difference is faster, is 300 − 5 = 295 s, printed 4:55.
The reverse: two minutes off a marathon is 120 ÷ 42.195 = 2.844 s/km, which is 2.844 × 1.609344 = 4.58 s/mi. Two minutes off a half is 120 ÷ 21.0975 = 5.688 s/km. The same two minutes costs 2.84 seconds per kilometre at the marathon and 5.69 at the half, which is the same fact read the other way round: a fixed time is worth less per kilometre the longer the race.
The lookup table, and how to read it
The first table above holds one row for each common difference from 1 to 30 seconds per kilometre, with the time it is worth over each of the four distances next to it. The row nearest the difference you typed is highlighted, so the table doubles as a way of reading the page at a glance rather than a separate reference you have to hunt through.
Two things about reading it. The columns are additive: the row for 10 s/km is exactly twice the row for 5 s/km, in every column, because the whole table is one multiplication. And the last column is not a separate calculation — it is the first column multiplied by 1.609344, so a reader who works in miles can start from the right-hand edge and read leftwards.
Seconds or percent: two ways to state one difference
The second table states the same differences as a share of a base pace. Both are honest; they answer different questions, and mixing them up is how a modest change gets described as a large one.
Seconds are additive. Five seconds per kilometre is worth 3:31 at the marathon whether you run that marathon at 4:00 per kilometre or at 7:00 per kilometre. Nothing in the first table depends on your pace.
A percentage is scale-free. The same five seconds is 1.67% of a 5:00 pace and 1.19% of a 7:00 pace. That makes percentages the right unit for comparing two runners whose paces differ, and the wrong unit for asking what a change is worth in a race.
One consequence worth saying out loud: a percentage that looks small can still be a large amount of time. One percent of a 5:00 per kilometre pace is 3 seconds per kilometre, which is 2:06 over a marathon. The table prints the seconds, the percent column prints the share, and neither is more true than the other.
Supported range and limits
- Pace difference: 0.1 to 600 seconds per kilometre or per mile. 600 s/km is ten minutes per kilometre, which is not a race pace; the ceiling exists so a mistyped figure is refused rather than multiplied.
- Base pace: 1:00 to 60:00, in the same unit as the difference. Used only for the percentage and the new pace.
- Distances: 5 km, 10 km, 21.0975 km, 42.195 km. The measured race lengths, not rounded.
- Time difference in the reverse block: up to 12:00:00 per distance.
- Rounding: times to the nearest second, percentages to two decimals, the per-mile figure to two decimals. One rounding, at the end, never halfway through a chain.
- Cost: none. No account, no email, no paywall, no daily limit, no request to a server.
- Privacy: every number is computed in your browser. Nothing you type is sent anywhere or stored — reload the page and it is gone.
- Works offline once the page has loaded, and on a phone.
What this page deliberately does not do
- No prediction of improvement. The page converts a difference you name; it does not say what difference you could produce. How much faster a training block makes you is a claim about your body and your history, and nothing here measures either. If you want a finish time predicted from a race you have already run, that is the race time predictor on the home page.
- No training plan. No weekly volume, no session design, no progression, no rest days. The page prices a difference; producing one is a different job and this site does not do it.
- No negative-split model. The arithmetic assumes the difference is held for the whole distance. A race where the halves are run differently is a shape, not a rate, and this page does not model one.
- No course, weather or altitude correction. A hilly course, a hot day and a race at altitude all change what a pace costs. This page prices none of them; the altitude pace calculator is the page that prices elevation.
- No fitness or heart-rate conversion. A pace difference is not converted into a VO2 max difference, a heart-rate difference or a power difference. Those need models with sources this page does not carry.
- No single-race pace, time and distance conversion. If you have two of those three for one race, the race pace calculator is the right page; this one is only about differences between two paces.
- No age or sex grading. A difference is not adjusted for either, and no table here is split by either.
Questions
How do I work out how much time a pace difference is worth?
Multiply the difference in seconds per kilometre by the race distance in kilometres. Five seconds per kilometre is 5 x 5 = 25 seconds over 5 km, 5 x 10 = 50 seconds over 10 km, 5 x 21.0975 = 105.5 seconds over the half marathon, and 5 x 42.195 = 211 seconds, or 3:31, over the marathon. The whole calculation is that one multiplication, and the page does it for all four measured distances at once.
Is 5 seconds per kilometre worth the same at every distance?
No, and that is the point of the page. A pace difference is a rate and a race time is an amount, so the amount grows with the distance. Five seconds per kilometre is 25 seconds at 5K and 3:31 at the marathon, which is eight times larger, because the marathon is 8.44 times as long. Nothing about the effort changes between the two rows; only the distance the rate is multiplied by.
How do I convert a seconds-per-mile difference to seconds per kilometre?
Divide by 1.609344, and multiply to go the other way. A mile is 1609.344 metres exactly, so one second per kilometre is 1.609344 seconds per mile and one second per mile is 0.621371 seconds per kilometre. Eight seconds per mile is 4.97 seconds per kilometre. The page has a unit selector for the difference and prints the equivalent in the other unit next to every answer, so neither direction has to be done by hand.
How much pace do I need to find to take two minutes off my marathon?
Divide the time you want to save by the distance: 120 seconds over 42.195 km is 2.844 seconds per kilometre, or 4.58 seconds per mile. The reverse block on the page does exactly that division for any of the four distances. What it does not do is tell you whether two minutes is available to you: the page converts the difference you name, and a difference you name is an assumption, not a finding.
Is a percentage of pace a better way to state a difference?
It answers a different question. Seconds are additive: the same 5 seconds per kilometre is worth 3:31 at the marathon whatever pace you run it at. A percentage is scale-free: 5 seconds is 1.67 percent of a 5:00 pace and only 1.25 percent of a 6:40 pace. Use seconds when you want to know what a change is worth in a race time, and a percentage when you want to compare two runners whose paces are different. The page prints both, from the base pace you type.
Does this page tell me how to get faster?
No. It converts a difference you already have in mind into the time that difference is worth, and it does not say how to produce the difference. Training structure, weekly volume, session design and recovery are outside what a converter can source, and this site does not publish a training plan. If you do not yet know what difference you can hold, this page will not tell you, and a number invented here would be a guess dressed as an answer.
Can I use this for a race I have already run?
Yes, as arithmetic on the result. If you ran 3:30:00 and you want to know what 3:28:00 implies per kilometre, type 2:00 into the marathon box in the reverse block and it returns 2.84 seconds per kilometre. What the page will not do is take 3:30:00 and predict what you could run next, because that is a claim about your training rather than a conversion of your numbers.
Why do the numbers assume even pacing?
Because a difference multiplied by a distance is only exact when the difference is held for the whole distance. Run the first half 10 seconds per kilometre faster and the second half at the old pace and you get half of the time the table promises. The table is the arithmetic of a difference held throughout, which is what a target pace means; it is not a model of how a race actually unfolds, and this page does not print one.