Swimming Formulas

Swimming is a numbers sport. Whether you are chasing a personal best, structuring a training set, or just curious how many calories your session burned, the same handful of formulas sit underneath every calculation. This page collects all of them in one place, shows exactly how each one works, and gives a worked example for each so you can check the maths by hand before trusting a calculator with it.

Every formula below is the exact equation our calculators apply. Where a unit conversion is needed, it is shown explicitly. Times are always handled in seconds internally and only formatted into minutes and seconds for display.

1. Pace (time per 100 m)

Pace is the single most useful number in pool swimming. It tells you how long it takes to cover a fixed reference distance — almost always 100 m or 100 yd — and it lets you compare efforts across different distances on a like-for-like basis. A 1500 m time of 22:30 is impressive, but converting it to a pace of 1:30 per 100 m makes it instantly comparable to a 100 m swim.

Example. You swim 400 m in 5:30. Convert the time to seconds: 5 × 60 + 30 = 330 s. Divide the distance by 100 to get the number of 100 m blocks: 400 ÷ 100 = 4. Now divide time by blocks: 330 ÷ 4 = 82.5 s. That is 1 minute and 22.5 seconds, written 1:22.5 per 100 m.

Use the pace calculator to do this for any distance and time.

2. Speed (metres per second)

Speed is the raw velocity of your swim, usually expressed in metres per second (m/s) or kilometres per hour (km/h). Pace and speed are two sides of the same coin — pace is time per distance, speed is distance per time — but speed is the more natural unit when you need to compare swimming with cycling or running, or when you are doing physics-based analysis such as drag calculations.

Example. You swim 100 m in 55 s. Speed = 100 ÷ 55 = 1.818 m/s. To convert to km/h, multiply by 3.6: 1.818 × 3.6 = 6.55 km/h. That is a strong age-group sprint pace; world-class freestylers exceed 2.0 m/s (7.2 km/h) over 100 m.

Use the speed calculator for any distance and time.

3. Time (how long a distance will take)

If you know your target pace and the distance you plan to swim, the time formula tells you how long it should take. This is the calculation you reach for when planning a race, estimating a finish time, or setting a goal for a training set.

Example. You want to swim 1500 m holding 1:30 per 100 m. Number of 100 m blocks = 1500 ÷ 100 = 15. Pace in seconds = 90. Time = 15 × 90 = 1350 s. Convert back: 1350 ÷ 60 = 22.5 minutes = 22:30. That is a solid Olympic-distance triathlon swim.

Use the time calculator to project finish times from a pace.

4. Distance (how far you swam in a given time)

The distance formula is the inverse of the time formula. Given a known pace and a known swim time, it tells you how far you travelled. This is useful when you have a pace target and a time budget — for example, “I have 30 minutes and I want to hold 1:45 pace, how far will I get?”

Example. You swim for 30:00 holding 1:45 per 100 m. Time in seconds = 1800, pace in seconds = 105. Distance = (1800 ÷ 105) × 100 = 17.14 × 100 = 1714 m. In a 25 m pool that is about 68.6 lengths.

Use the distance calculator for this calculation.

5. CSS — Critical Swim Speed

Critical Swim Speed (CSS) is the pace you can theoretically sustain indefinitely without rapid fatigue — roughly the threshold between aerobic and anaerobic effort. It is the swimming equivalent of a running lactate threshold pace, and it is the backbone of modern, pace-based swim training. CSS is derived from two maximal time trials: a 200 m and a 400 m. The slope of the line between those two points gives your sustainable speed.

Example. Your 200 m time trial is 2:30 (150 s) and your 400 m time trial is 5:15 (315 s). CSS = 200 ÷ (315 − 150) = 200 ÷ 165 = 1.212 m/s. To express this as a pace per 100 m: 100 ÷ 1.212 = 82.5 s = 1:22.5 per 100 m. That is your CSS pace, and it is the anchor for every training zone (see the training zones guide).

Use the CSS calculator to compute CSS from your own time trials.

6. SWOLF — efficiency score

SWOLF (swim + golf) is a swimming efficiency score borrowed from golf, where lower is better. You add the time in seconds and the stroke count for a fixed distance — usually 25 m or 50 m — and the sum is your SWOLF. A lower score means you are covering the distance with less time and fewer strokes, which is the definition of efficient swimming.

Example. You swim 25 m in 22 s taking 18 strokes. SWOLF = 22 + 18 = 40. For a 25 m pool, SWOLF in the low 40s is competent adult fitness swimming; sub-35 is strong; world-class open-water swimmers can score in the high 20s over 25 m. Track SWOLF over weeks at a fixed distance and you have a direct, objective measure of whether your technique is improving.

Use the SWOLF calculator to score a swim.

7. Stroke rate (strokes per minute)

Stroke rate, often abbreviated SR and measured in strokes per minute (SPM), is the cadence of your swimming — the swimming equivalent of cycling cadence or running stride rate. A higher stroke rate moves you faster but costs more energy; a lower rate is more efficient but may be too slow to race at. Most efficient freestylers sit between 30 and 50 SPM in steady swimming, with sprinters reaching 70 SPM or more.

Example. You count 15 strokes over a 30-second 50 m repeat. SPM = (15 ÷ 30) × 60 = 0.5 × 60 = 30 SPM. That is a comfortable, long-stroke tempo. Double the cadence to 60 SPM and you are sprinting.

Use the stroke rate calculator to find your cadence.

8. Stroke length (distance per stroke, DPS)

Distance per stroke (DPS) is how far you travel on a single stroke cycle. It is the complement of stroke rate: DPS × stroke rate = speed. Improving DPS — travelling further on each stroke — is the primary technique goal for most adult swimmers, because a longer, more efficient stroke lets you swim faster at the same effort. DPS is typically reported in metres per stroke.

Example. You swim 50 m taking 36 strokes. DPS = 50 ÷ 36 = 1.39 m/stroke. For freestyle, a DPS above 1.5 m/stroke is generally considered good for adult swimmers; elite distance freestylers can exceed 2.0 m/stroke. Note that some watches count an arm cycle (both arms) as one stroke, which doubles the reported DPS — be consistent with how you count.

Use the stroke length calculator to work out your DPS.

9. Splits (even pacing)

A split is the time for a segment of a swim — each 50 m of a 200 m race, or each 100 m of a 400 m set. Even splits mean each segment is swum in the same time, which is the most efficient way to pace distances from 200 m up. The split formula simply divides total time by the number of segments, giving you the target time per split.

Example. You want to swim 400 m in 5:20 (320 s) with even splits across four 100 m repeats. Split = 320 ÷ 4 = 80 s = 1:20 per 100 m. Hitting 1:20, 1:20, 1:20, 1:20 is a textbook even-split swim and usually faster than going out hard and fading.

Use the split calculator to plan even or target splits.

10. Intervals and send-offs

In interval training, each repeat is swum on a fixed send-off — the clock time you leave on each rep. The send-off equals the expected swim time plus the rest you want. The total set time is the send-off multiplied by the number of reps. Getting the send-off right is what makes a set hit the intended energy system: too slow and it becomes easy aerobic work; too fast and you blow up halfway through.

Example. You swim 8 × 100 m at CSS pace of 1:30 with 10 s rest. Send-off = 90 + 10 = 100 s = 1:40. Total set time = 100 × 8 = 800 s = 13:20. You leave on 1:40, 3:20, 5:00 and so on. If you swim faster than 1:30 you get more rest; if you slow down, your rest shrinks — the send-off automatically scales the difficulty.

Use the interval calculator to build send-off sets.

11. Calories burned

Calorie expenditure in swimming is estimated using MET (metabolic equivalent of task) values. One MET is the energy you burn at rest; swimming at a given effort is assigned a MET value, and the total calories depend on your body weight and the duration. The formula is simple, but the accuracy hinges entirely on choosing the right MET for your intensity and stroke.

Example. A 70 kg swimmer does 30 minutes (0.5 h) of moderate freestyle, which has a MET of 8.3. Calories = 8.3 × 70 × 0.5 = 290.5 kcal. The same swimmer doing a hard butterfly set (MET 13.8) for the same 30 minutes would burn 13.8 × 70 × 0.5 = 483 kcal — nearly 70% more for the same clock time, because the effort is far higher.

Use the calorie calculator, which picks the correct MET for your stroke and intensity.

MET values for common swimming activities

The table below lists MET values from the Compendium of Physical Activities. Pick the row that matches your stroke and effort — overestimating the intensity is the most common source of inflated calorie numbers.

12. Unit conversions

Most swimming confusion comes from mixing metres and yards. The conversions below are the exact factors our calculators use. The key numbers to remember: a metre is about 9% longer than a yard, so a 25 m pool is roughly 27.3 yd and a 25 yd pool is about 22.9 m.

A few useful swim-specific conversions: 100 m = 109.36 yd, so a 100 m time is not directly comparable to a 100 yd time — the yard swim is about 8.4% shorter. The “mile” also differs: a statute mile is 1609.34 m, but the US competitive swimming mile is 1650 yd (1508.8 m), and a “pool mile” is often rounded to 1500 m or 1600 m. Always check which mile a watch or race is using.

For any distance conversion, use the metres to yards converter; for converting lengths to distance, use the pool length calculator. The full background on every unit is in the swimming units guide.

Putting the formulas together

These formulas are not isolated — they chain together. A typical training session uses all of them: you start from a CSS test, convert that into training zones, plan a set with send-offs, check your pacing with splits, track technique with SWOLF and DPS, and estimate the energy cost with the calorie calculator. Understanding the maths behind each one means you can sanity-check every result and, more importantly, design your own sets with confidence.

Every calculator on this site applies the exact formulas above. Pick one from the sidebar and try it with your own numbers.