The first half marathon I ran, I went out too fast. I had done the training but never done the arithmetic, so I had no idea what pace my goal time actually required, and I paid for it in the last five kilometres when the pace I had been holding turned out to be thirty seconds per kilometre faster than I could sustain. A pace calculator would have told me the exact number to aim for before the gun, and a split table would have given me checkpoints to hold myself to. That is the whole job of the Pace Calculator on toolz.dev: it turns the relationship between distance, time, and pace into a single number you can carry to the start line. This guide covers how pace works, how to use the tool for planning and reviewing runs, and why even splits beat a fast start almost every time.
TL;DR: Pace is time divided by distance, usually minutes per kilometre or per mile. Give the Pace Calculator any two of distance, time, and pace and it computes the third, plus speed in both units and a per-kilometre or per-mile split table. Everything runs in your browser with nothing uploaded, so it works at the start line even with no signal.
What is a pace calculator?
A pace calculator works out the relationship between how far you go, how long it takes, and how fast you move. Those three quantities are locked together by one equation - and by the SI definitions of the metre and the second underneath it - pace equals time divided by distance, so if you know any two the third is fixed. Enter a 10K and a 55-minute finish and the tool returns 5 minutes 30 seconds per kilometre. Enter a target pace and a distance and it returns the finish time. Enter a time and a pace and it returns how far you would travel. One tool covers race prediction, goal planning, and workout review, because they are all the same equation solved for a different unknown.
Pace and speed are the same information pointed in opposite directions, and confusing them is one of the most common sources of error. Pace is time per distance, minutes per kilometre or per mile, and it is how runners think, because it maps directly to what a watch shows on each split. Speed is distance per time, kilometres or miles per hour, and it is how cyclists and treadmills label effort. A faster runner has a lower pace number but a higher speed number, which is why looking at the wrong one gives a backwards impression. The calculator shows both at once so you can read whichever your device or plan uses.
On toolz.dev the flow is quick. You choose what to calculate, enter the two values it needs, and the result appears with pace in both units, speed in both units, and a split table. Race presets fill in the exact official distances with one tap, so you do not have to remember that a marathon is 42.195 kilometres rather than a round 42.
What is the difference between pace and speed?
They measure the same motion but invert each other, and the inversion is the source of most confusion. Pace answers "how long does one unit of distance take," so a smaller number is faster. Speed answers "how much distance in one unit of time," so a larger number is faster. A runner at 5:00 per kilometre is moving at 12 kilometres per hour; improve to 4:00 per kilometre and the pace number falls while the speed number rises to 15.
The reason both survive is that different activities settled on different conventions. Running culture is built around pace because a running watch beeps every kilometre or mile and shows the split, so runners plan and pace themselves in minutes per unit. Cycling and gym equipment are built around speed because a bike computer and a treadmill display a rate, and a cyclist thinks in kilometres per hour without ever converting to pace. Neither is more correct; they are two dialects for the same thing.
The practical problem is comparing efforts across the two dialects, or reading a plan written in the other unit. A training plan in minutes per mile is opaque to someone who trains in minutes per kilometre until it is converted, and a treadmill speed is meaningless to a runner who thinks in pace. The calculator solves this by reporting all four figures on every result, pace per kilometre, pace per mile, speed in kilometres per hour, and speed in miles per hour, so a plan in any unit is instantly readable and no mental conversion is needed.
How do I convert between min/km and min/mile?
The conversion rests on one fixed number: a mile is 1.609344 kilometres. To turn a per-kilometre pace into a per-mile pace, multiply by that factor, because a mile contains more kilometres and therefore takes proportionally longer. A 5:00 per kilometre pace becomes about 8:03 per mile. To go the other way, divide. The arithmetic is simple in principle but fiddly in practice, because pace is written in minutes and seconds rather than decimal minutes, so the conversion means converting to seconds, multiplying, and converting back.
That last step is where hand calculation goes wrong. A 4:45 per kilometre pace is not 4.45 minutes; it is 4.75 minutes, or 285 seconds. Multiply by 1.609344 to get roughly 459 seconds, then convert back to 7 minutes 39 seconds per mile. Miss the minutes-to-seconds conversion and you get a nonsense answer that is off by a wide margin. The calculator handles the minutes-and-seconds bookkeeping automatically and shows both units on every result, so the conversion is never something you do by hand.
Here is a quick reference for common paces, the kind of table I used to keep in a training notebook:
| Pace /km | Pace /mile | Speed km/h | Speed mph |
|---|---|---|---|
| 4:00 | 6:26 | 15.00 | 9.32 |
| 4:30 | 7:14 | 13.33 | 8.28 |
| 5:00 | 8:03 | 12.00 | 7.46 |
| 5:30 | 8:51 | 10.91 | 6.78 |
| 6:00 | 9:39 | 10.00 | 6.21 |
| 6:30 | 10:28 | 9.23 | 5.73 |
For converting the underlying distances rather than paces, a general-purpose unit converter handles kilometres, miles, metres, and yards, which is useful when a route is measured in one unit and your plan is in another.
How accurate are the race distance presets?
The presets use the official distances, and using the exact figures matters more than it looks. A 5K is 5 kilometres and a 10K is 10, both round numbers, but the half marathon is 21.0975 kilometres and the full marathon is 42.195 kilometres, and rounding those to 21 and 42 introduces real error. At a typical five-minute-per-kilometre pace, rounding the marathon down by 0.195 kilometres understates the finish time by nearly a full minute, which is exactly the margin that separates hitting a goal from missing it.
The marathon distance itself has a specific history. It was fixed at 42.195 kilometres, or 26 miles and 385 yards, at the 1908 London Olympics and formally standardized by the governing body of the sport, World Athletics, so it is not an arbitrary round number and should never be treated as one when the goal time is tight. The half is exactly half of that, 21.0975 kilometres, which is why it carries such an awkward decimal.
The presets on the calculator fill in these exact values with a single tap, in whichever unit you have selected, so you get the correct finish time without having to remember the decimals or convert them yourself. For any non-standard distance, a parkrun of a different length or a training route you measured, you simply type the distance in and the same math applies.
How do splits help me race better?
Splits are the cumulative times you should hit at each kilometre or mile to finish on a target pace, and they turn an abstract goal into a series of concrete checkpoints. Instead of trying to feel whether you are on pace, which is unreliable especially early when everything feels easy, you glance at your watch at each marker and compare it to the plan. If your 5-kilometre split says you should be at 25:00 and your watch says 24:20, you know you are running too fast and can ease off before it costs you later.
The reason this works is that even pacing, running each segment at roughly the same pace, is one of the most reliable ways to race well. Starting too fast feels great for the first few kilometres and then extracts a heavy toll in the final ones, where a pace that was thirty seconds too quick early can cost minutes late. This is precisely the mistake I made in my first half marathon, and the split table exists to prevent it. Almost every well-executed distance race is run on even or slightly negative splits, where the second half matches or beats the first.
The calculator generates the full split table automatically from your distance and pace, listing the cumulative time and the individual split time for each kilometre or mile, plus a final partial split when the distance is not a whole number of units, as with the half and full marathon. You can copy the table into a training log or write the key checkpoints on your arm before a race, which is the low-tech version runners have used for decades. For structured workouts that combine running blocks with rest intervals, the time duration calculator complements this by summing and manipulating individual time segments.
When would I actually use this?
Race planning is the primary case. Before a goal race you work backwards from the finish time to the pace it requires, then commit that pace and its splits to memory. Selecting the marathon preset, choosing to solve for pace, and entering a four-hour target tells you immediately that you need about 5:41 per kilometre, and the split table gives you the checkpoint at every kilometre so you can hold it. Doing this arithmetic before the race rather than during it is the difference between running your plan and improvising.
Workout review is the mirror image. After a run you have the distance and the time, so you solve for pace to see how the session actually went, then compare it to the target. Over weeks this builds a picture of your fitness trend that a single run cannot show. Tracking that trend alongside other numbers, resting metrics, weight, training load, is part of a broader picture, and tools like the TDEE calculator and BMI calculator sit in the same corner of my bookmarks for the health-and-fitness side of training.
Cross-training and general planning round it out. The same math works for walking and cycling, so a walker can find the pace for a timed event and a cyclist can read the speed figures directly. Planning a long training block, estimating how far a time-limited run will take you, or converting a coach's plan from miles to kilometres are all quick jobs for the tool. I average sets of runs to spot trends with the average calculator, and the whole cluster of everyday calculators is the kind of quiet productivity kit I wrote about in the developer productivity tools guide. The broader case for keeping small tools like this in the browser rather than in apps is one I made in the web developer toolkit guide.
Is my training data private?
Yes. Every calculation runs in JavaScript inside your browser. Your distances, times, and paces are never transmitted, logged, or stored, and the tool keeps working with no network connection once the page has loaded. That last point is not academic for runners: start lines and trailheads are exactly the places with the worst signal, and a calculator that needs a connection is useless there. Because this one does all its work locally, you can pull it up at the start of a race, confirm your target pace and splits, and put your phone away, all without a single byte leaving your device. It is the same privacy-first approach behind every tool on toolz.dev.
FAQ
How do I calculate my running pace?
Divide your total time by the distance you covered. For a 10-kilometre run in 55 minutes, the pace is 55 divided by 10, which is 5.5 minutes per kilometre, or 5 minutes 30 seconds per km. The calculator does the division and the minutes-and-seconds conversion for you, and also gives the equivalent per-mile pace and your speed in both units.
What is the difference between pace and speed?
Pace is time per distance, such as minutes per kilometre or per mile, while speed is distance per time, such as kilometres or miles per hour. They describe the same effort inversely: a faster runner has a lower pace number but a higher speed number. This tool shows both so you can read whichever your watch, treadmill, or plan uses without converting.
How do I convert min/km to min/mile?
Multiply your per-kilometre pace by 1.609344, the number of kilometres in a mile, to get the per-mile pace, because a mile is longer and takes proportionally more time. A 5:00 per km pace is about 8:03 per mile. The calculator reports both units on every result, so you never have to do this conversion by hand or juggle the minutes-and-seconds arithmetic.
What pace do I need for a sub-4-hour marathon?
A marathon is 42.195 km, so a 4-hour finish needs an average pace of about 5:41 per kilometre, or 9:09 per mile. Select the Marathon preset, choose to solve for pace, and enter 4 hours to see the exact figure. It is wise to train slightly faster than target so you have a margin for the final miles, where fatigue makes holding pace harder.
How accurate are the race presets?
The presets use the official distances: 5 km, 10 km, 21.0975 km for the half marathon, and 42.195 km for the marathon. Using the exact half and full marathon distances matters, because rounding the marathon to 42 km understates the finish time by nearly a minute at typical paces, which is often the margin between hitting and missing a goal.
What are splits and why do they matter?
Splits are the cumulative times you should hit at each kilometre or mile to finish on target. Following even splits, running each segment at the same pace, is one of the most reliable ways to race well, since starting too fast usually costs more time late than it gains early. The split table gives you those checkpoints so you can hold pace instead of guessing.
Does this work for walking and cycling too?
Yes. The math is identical for any steady activity: enter distance and time to get pace and speed, or enter a target pace to find a finish time. Walkers often think in minutes per kilometre like runners, while cyclists usually prefer the km/h and mph speed figures, both of which the tool shows on every result.
Is my training data private?
Yes. Every calculation runs in your browser in plain JavaScript. Your distances, times, and paces are never transmitted, logged, or stored, and the tool keeps working with no network connection once the page has loaded, which is exactly what you need at a start line with poor signal.
Plan your next run with the free Pace Calculator. It computes pace, time, or distance plus speed and a full split table, entirely in your browser, with nothing uploaded.
