How every number on this site is worked out

Pace, time and distance

Everything starts from one identity: time = pace × distance ÷ 100, where pace is the time taken to cover 100 units (metres or yards). Rearranged, it gives the three modes of the swim pace calculator:

• Speed: a pace of P seconds per 100 m is 360 ÷ P km/h (100 m in P seconds is 0.1 km in P⁄3600 hours). Per 100 yd, speed in mph is 204.5 ÷ P.
• Metres ↔ yards: 1 yard is exactly 0.9144 metres (International Yard and Pound Agreement, 1959; NIST SP 811). So a pace per 100 m converts to per 100 yd by multiplying by 0.9144, and back by dividing.
• Send-offs: send-off = swimmer's pace per repeat + chosen rest, rounded up to the next 5 seconds, because pace clocks are read in 5-second steps.

Worked check: 1500 m in 30:00 → 1800 ÷ 1500 × 100 = 120 s = 2:00 per 100 m; in yards, 120 × 0.9144 = 109.7 s ≈ 1:50 per 100 yd; speed 360 ÷ 120 = 3.0 km/h; send-off with 15 s rest = 2:15.

Split plans

The split calculator distributes a goal time across equal segments three ways. Even splits are the goal pace repeated. Negative split and fast start plans ramp each segment's pace linearly from 3% off the average at the first segment to 3% the other way at the last, then normalise the whole plan so the segments sum to the goal time exactly. That ±3% linear model is this site's own planning convention — it is stated on the page, not borrowed from a governing body, because no governing body publishes one.

Worked check: 400 m in 4:48, negative split, per-100 segments: factors 1.03 / 1.01 / 0.99 / 0.97 over an average of 72 s give 1:14.2, 1:12.7, 1:11.3, 1:09.8 — summing to 4:48.0.

Course conversion (SCY / SCM / LCM)

There is no single official conversion system, and any site that prints one tidy per-stroke factor table without naming its system is inventing it. Two real systems exist, for two different jobs:

The system this site uses: USA Swimming / Hy-Tek

The conversion calculator implements the factor-plus-increment method published by SportsEngine Motion — the team platform whose factors (the Hy-Tek defaults) USA Swimming teams use for meet entries. Times are handled as:

• SCY → SCM or LCM: converted = time × factor + increment
• SCM or LCM → SCY: converted = (time − increment) ÷ factor
• SCM → LCM: converted = time + increment  ·  LCM → SCM: converted = time − increment

The factor is 1.11 for yards ↔ metres at the same nominal distance; 1.0 between SCM and LCM; 0.8925 for the distance-changing freestyle pairs 400 m ↔ 500 yd and 800 m ↔ 1000 yd when yards are involved; 0.997 for 1650 yd ↔ 1500 m between yards and SCM, and 1.02 for the same pair between LCM and SCY.

The increment prices the turns gained or lost between a 25-unit and a 50-unit course. Per 50 of the event it is: butterfly 0.70 s, backstroke 0.60 s, breaststroke 1.00 s, freestyle 0.80 s, individual medley 0.80 s — scaled ×1 for a 50, ×2 for a 100, ×4 for a 200, and 6.4 s for the 400 IM. Between SCM and LCM at the distance freestyle pairs it is 6.4 s (400/500), 12.8 s (800/1000) and 24.0 s (1500/1650). Between SCY and SCM the increment is always zero — the 1.11 factor already carries the difference.

Worked checks: 100 m freestyle SCM 1:00.00 → SCY = 60 ÷ 1.11 = 54.05. The same swimmer's 54.05 SCY → LCM = 54.05 × 1.11 + 1.6 = 1:01.60. A 5:00.00 500 yd freestyle → SCM (400 m) = 300 × 0.8925 = 4:27.75.

The other system: NCAA championship factors

For college championship seeding, the NCAA publishes its own multiplicative table (the conversion calculator shows it as a comparison whenever you convert SCM → SCY). In the 2016 Division I table, SCM → SCY is a single factor of 0.906 for every event — no per-stroke variation — except the distance freestyle pairs: 400 m → 500 yd and 800 m → 1000 yd at 1.153, and 1500 m → 1650 yd at 1.013. The NCAA re-issues its factors between championship years; the 2011 handbook's own worked example uses a different men's 200 m breaststroke factor (0.858) from the 2016 table (0.868). Neither system is "wrong" — they answer to different authorities, which is exactly why this site names the one it uses.

Critical Swim Speed (CSS)

Critical velocity comes from Wakayoshi and colleagues' 1992 research, which defined it as the slope of the distance–time line across several maximal swims and showed it tracks the lactate threshold. The two-swim version used by the CSS calculator — maximal 400 m and 200 m time trials in one session, push starts, full recovery between — is the standard field simplification of that model, popularised for adult swimmers by Swim Smooth. CSS velocity in m/s is 200 ÷ (T400 − T200); the pace per 100 m follows directly. Worked check: T400 6:00 and T200 2:48 → (360 − 168) ÷ 2 = 96 s = 1:36 per 100 m, at 1.04 m/s.

Training zones: there is no scientific or governing-body standard for zones derived from CSS. The zone table on the CSS page (recovery at CSS +15 s per 100, base +8, threshold at CSS, speed −6, sprint −12) is a widely used coaching convention and is labelled as such on the page. Swim Smooth's own published set prescriptions use CSS +8 / +6 / +4 / +2 / +0 s per 100 m across descending distances — also shown on the page, attributed.

Calories

MET values are taken from the 2024 Adult Compendium of Physical Activities (Herrmann et al., Journal of Sport and Health Science, 2024), water-activities table: freestyle laps fast/vigorous 9.8 (code 18230), freestyle laps slow/recreational 5.8 (18240), crawl medium speed 8.0 (18290), backstroke training 9.5 (18250) and recreational 4.8 (18255), breaststroke training 10.3 (18260) and recreational 5.3 (18265), butterfly 13.8 (18270), leisurely swimming 6.0 (18310), sidestroke 7.0 (18320), treading water fast 9.8 (18340) and moderate 3.5 (18350). Worked check: 70 kg at MET 9.8 for 45 minutes = 9.8 × 70 × 0.75 = 514.5 ≈ 515 kcal. These are population averages — an efficient swimmer burns measurably less at the same pace than a beginner, and the calculator says so.

Laps, lengths and the "pool mile"

This site counts a lap as one pool length and says so on the lap calculator. Distances use the exact definitions 1 yd = 0.9144 m and 1 mile = 1760 yd = 1609.344 m. The American "pool mile" is the 1650 yd freestyle — the SCY counterpart of the 1500 m freestyle, called a mile by convention. It is not a true mile: 1650 yd is 1508.76 m, which is 100.58 m (about 6%) short of 1609.344 m. Worked check: 66 lengths of a 25 yd pool = 1650 yd = 1508.76 m; a true mile takes 70.4 lengths of that pool.

IRONMAN swim paces

Distances and cutoffs are taken from current official IRONMAN athlete guides: the full-distance swim is 3.8 km with a 2 h 20 min swim cutoff; the 70.3 swim is 1.9 km with a 1 h 10 min cutoff. (IRONMAN labels these 2.4 and 1.2 miles; 3.8 km is exactly 2.36 miles, so the metric figures govern.) Both cutoffs imply the same slowest survivable pace: 8400 s ÷ 38 = 4200 s ÷ 19 = 3:41.1 per 100 m. Worked check: a 1:10:00 full-distance goal needs 4200 ÷ 38 = 110.5 s = 1:50.5 per 100 m, leaving a 70-minute buffer under the cutoff. Overall race cutoffs vary by event and are not used anywhere on this site — only the swim cutoffs, which are uniform across the official guides checked.

Open water

No governing body publishes an official open-water pace adjustment, and this site does not pretend otherwise. The open water calculator presents adjustments (+5% calm, +8% typical, +12% rough, custom to +20%, wetsuit −3 points) explicitly as coaching planning estimates — the range coaches commonly quote is 5–15% slower than pool pace. The one peer-reviewed number in this area is narrower than the folklore: a 2022 systematic review (Gay et al., Physiologia) found full-body wetsuits raised swimming velocity 3.2–12.9% versus swimsuits across the studies reviewed. That is a wetsuit-versus-swimsuit figure, not an open-water conversion, and the page presents it only as that.

How the calculators are verified

Every worked check on this page is reproduced by an automated test that runs the site's own calculator code against these exact inputs before any version is published. If a formula, factor or MET value is ever corrected, the correction is dated on the editorial policy page and announced on the page it affects. Found a number you can prove wrong? Report an error — corrections from readers are part of the method, not an embarrassment.

Sources