Pool to Open Water Swim Calculator
Turn your pool pace into an open water time range for your race.
Swimming in a lake or the sea is slower than in the pool: there are no walls to push off, you have to sight, and the water moves. This calculator starts from your CSS pace or a recent pool time trial, adds the effect of your conditions factor by factor, and gives you a range of times instead of one falsely precise number. It is built for triathletes and open water swimmers, from a 750 m sprint to a 10 km swim.
Your pool level and your race
Your pool level
Format m:ss (1:45), 1'45 or seconds (105), per 100 of the unit chosen above. Do not know your CSS? Work it out from two time trials
Race distance
Race distances are always in meters.
Race conditions
Pick what you expect on the day. Keep the defaults if you do not know yet.
A current helps one way and costs the other, the net cost is what counts.
Good means a quick look every few strokes and a straight line. Poor means zigzagging.
Some means swimming on feet or at a hip for part of the race, most means almost all of it.
A mass start means a crowded first minutes. A rolling start spreads the swimmers out.
Whole number, 0 to 20. Count the turns, not the buoys you only pass.
Enter your pool level and choose a distance to see your open water time range.
Why open water is slower than the pool
In a pool, every wall gives you a push and a glide, the lane rope keeps you straight and the black line shows the way. Take that away and the same effort gives a slower average speed. A calm lake already costs between 2 % and 3 % of your time just for the missing walls.
Then comes sighting. Each time you lift your head to find the next buoy you lose speed, and you drift a little off the straight line, so you swim more than the official distance. Waves, chop and current add to it, and the crowd of a mass start costs a few seconds at the beginning. That is why this calculator adds the factors one by one instead of applying a single magic percentage.
How much a wetsuit helps
A wetsuit lifts your legs and lowers drag, so you swim faster for the same effort. The gain is between 2 % and 4 % of your time for a swimmer holding 1:15 per 100 m, and between 5 % and 8 % for a swimmer at 2:00 per 100 m.
Slower swimmers gain more because they sit lower in the water and lose more to drag, while strong swimmers already hold a good body position. The calculator moves smoothly between the two according to your CSS pace. Check the race rules too: most races only allow wetsuits below a set water temperature.
Drafting in open water
Swimming on the feet or at the hip of another swimmer lowers the drag you push through, so you go faster or spend less energy at the same speed. Drafting for part of the swim is worth between 1 % and 3 % of your time, and staying on feet for almost the whole swim between 3 % and 6 %. Only count on it if you are sure to find swimmers at your pace.
How to use the range for race pacing
Do not aim for one end of the range. The fast end assumes every condition goes your way, the slow end assumes the worst. Plan your race around the middle, use the fast end as the goal for a good day and the slow end as the time to expect when conditions turn bad.
Start the first minutes a little below your pool effort, because a mass start and a crowded first buoy waste energy. Then settle on the open water pace shown above. It is slower than your pool pace because the water costs speed, not because you are swimming easier. After the race, compare your time with the range and update your conditions to see what really changed.
How the estimate is built
The calculator works in two steps. First it reads your pool level from the swim race predictor table, which turns a CSS pace into a pool time range for each distance, and extends it to any distance between 300 and 25000 m. A pool time trial is first converted into the CSS pace that predicts the same time.
Second, each condition adds or removes a share of your pool time, and the start and the turn buoys add a few seconds. The fast end of the range takes the best case of every factor on the fast pool time, the slow end takes the worst case on the slow pool time. The outer limits are then rounded outward to the next 15 seconds.
| Factor | Choice | Size |
|---|---|---|
| No walls to push off | +2 % to +3 % | |
| Sighting and course | Good | +2 % to +3 % |
| Sighting and course | Average | +3 % to +6 % |
| Sighting and course | Poor | +6 % to +12 % |
| Wetsuit | Wetsuit at 1:15 per 100 m | -4 % to -2 % |
| Wetsuit | Wetsuit at 2:00 per 100 m | -8 % to -5 % |
| Water | Lake | 0 % |
| Water | Sea | -1 % to 0 % |
| Chop | Calm | 0 % |
| Chop | Light chop | +2 % to +5 % |
| Chop | Rough | +5 % to +12 % |
| Current | None | 0 % |
| Current | Moderate | +1 % to +4 % |
| Current | Strong | +3 % to +10 % |
| Drafting | None | 0 % |
| Drafting | Some | -3 % to -1 % |
| Drafting | Most of the swim | -6 % to -3 % |
| Start | Rolling start | — to 0:10 |
| Start | Mass start | 0:15 to 0:45 |
| Turn buoys | each | 0:03 to 0:08 |
These sizes are prudent ranges drawn from the wetsuit, drafting and open water research listed below. They are estimates, which is why every line is shown.
Related Tools
Scientific References
- Toussaint HM, Bruinink L, Coster R, et al. (1989). Effect of a triathlon wet suit on drag during swimming. Medicine and Science in Sports and Exercise, 21(3):325-328.
- Cordain L, Kopriva R (1991). Wetsuits, body density and swimming performance. British Journal of Sports Medicine, 25(1):31-33.
- Chatard JC, Senegas X, Selles M, Dreanot P, Geyssant A (1995). Wet suit effect: a comparison between competitive swimmers and triathletes. Medicine and Science in Sports and Exercise, 27(4):580-586.
- Chatard JC, Wilson B (2003). Drafting distance in swimming. Medicine and Science in Sports and Exercise, 35(7):1176-1181.
- Baldassarre R, Bonifazi M, Zamparo P, Piacentini MF (2017). Characteristics and challenges of open-water swimming performance: a review. International Journal of Sports Physiology and Performance, 12(10):1275-1284.
Frequently Asked Questions
How much slower is open water than the pool?
For most swimmers open water is roughly 5 to 10 percent slower than the same effort in a pool when it is calm and the course is straight, before any wetsuit. The missing walls explain part of it and sighting most of the rest. Chop, current, a mass start and a poor line push it higher. The calculator shows each factor separately for your own conditions.
How much faster is a wetsuit in open water?
A wetsuit typically saves about 2 to 4 percent for a strong swimmer and about 5 to 8 percent for a slower one, because it lifts the legs and cuts drag. Over a 70.3 swim that is between one and three minutes. The calculator adjusts the gain to your CSS pace.
How do I convert my pool pace to an open water pace?
Enter your CSS pace per 100 (or a recent 400 or 1500 pool time trial), pick the race distance and describe your conditions. The calculator returns the open water pace as a range per 100 m or per 100 yd, which you can use as your pacing target. As a quick rule, add 5 to 10 percent to your pool pace in calm water, then adjust for the wetsuit and the conditions.
What is a good open water swim time for a 70.3 or an Ironman?
It depends on your level. A swimmer with a CSS of 1:45 per 100 m should expect about 34 to 40 minutes over 1900 m and 1:10 to 1:22 over 3800 m, depending on the wetsuit and the conditions. At 2:00 per 100 m the same races take about 37 to 46 minutes and 1:18 to 1:36. Enter your own CSS and conditions for your range.
Does drafting help in a triathlon swim?
Yes. Swimming on the feet or at the hip of another swimmer lowers drag, so you save energy or go faster. Even partial drafting is worth a few percent over a longer swim, and it also helps with sighting because you follow someone else. Drafting is legal in the swim, but you need swimmers at your pace to benefit.