Swimming Interval Calculator
Build structured swim sets. Enter your repeat distance, pace, rest and number of reps to get your send-off interval and total set time.
What is a swimming interval?
An interval is the repeating cycle of a structured swim set: swim a fixed distance, rest for a fixed time, then repeat. The set 10 × 100 m on 1:45 means you swim ten 100 m repeats, each one leaving every 1 minute 45 seconds on the pace clock. The send-off (1:45) is the heart of the set — it fixes the rhythm and, combined with your swim pace, determines how much rest you get.
Intervals are the building block of almost every training workout because they let a coach control two things at once: the intensity (how fast you swim each repeat) and the density (how much rest you get between them). Tweaking either changes the physiological stimulus of the set entirely.
How send-offs work
A send-off is expressed as a time per repeat, written as on X:XX. When the coach says 10 × 100 on 1:45, the pace clock drives everything: the first repeat leaves at 0:00, the second at 1:45, the third at 3:30, and so on. If you finish a repeat in 1:30, you earn 15 seconds rest. Finish in 1:40 and you get just 5 seconds. Finish slower than 1:45 and you have already missed the next send-off — a clear signal the set is too aggressive for your current pace.
This is why send-offs are so effective: they are self-correcting. Faster swimming is rewarded with more rest; slower swimming is punished with less. The same send-off trains different swimmers at their own individual intensity, which is invaluable in a lane of mixed ability.
repeat time = (distance ÷ 100) × pace per 100
send-off = repeat time + rest
total set time = send-off × reps
total distance = repeat distance × reps
How rest shapes the training stimulus
Rest is the single biggest lever for changing what a set achieves. The same 10 × 100 m becomes a completely different workout depending on the rest:
- Very short rest (5–10 s) — heart rate stays high, lactate accumulates. This trains aerobic endurance and lactate tolerance. Brutal but effective for distance racing.
- Moderate rest (15–20 s) — the sweet spot for threshold / CSS work. You stay just under the red line, raising the pace you can sustain. Use the CSS calculator to find that pace.
- Long rest (30–60 s+) — near-full recovery between repeats. Used for race-pace and sprint sets where the goal is speed quality, not fatigue resistance. Think 8 × 50 m on 1:00 at 200 m race pace.
Common swim sets and what they build
Here are some archetypal interval sets. Plug any of them into the calculator to see the send-off and total time:
- 10 × 100 m on 1:30 — a classic aerobic/threshold set. With a 1:20 swim time you get 10 s rest; the set takes 15:00 total.
- 8 × 50 m on 1:00 — short, sharp race-pace work. Each 50 at 35 s leaves 25 s rest. Great for 100 m and 200 m speed.
- 5 × 200 m on 3:00 — longer threshold repeats. Builds the endurance to hold CSS pace over distance.
- 20 × 50 m on 0:45 — high-density sprint/aerobic work. Very little rest, high heart rate, big lactate stimulus.
Most pools use a 60-second rolling pace clock with a red second hand. Send-offs are timed off that hand, not a stopwatch. If your send-off is 1:45, leave when the red hand is at a fixed mark (say 12 o'clock) every time it returns there — which happens every 60 seconds for a 1:00 send-off, or every 1:45 if you use a secondary marker. Ask a lane-mate to count you down if you are new to it.
Building a full workout
A complete session is usually a warm-up, a pre-set (drills or kicks), a main set of intervals, and a swim-down. Use this calculator to design the main set, then layer supporting work around it. Pair it with the pace calculator to translate your CSS into per-100 targets, and the training zones guide to make sure the intensity matches the day's goal. For breaking a single target time into per-100 splits, use the split calculator.
Related calculators
Swimming Interval Calculator FAQ
A send-off interval is the total cycle time for one repeat in a set, including the swim time and the rest. When a coach says 10 × 100 m on 1:45, the 1:45 is the send-off. You leave every 1 minute 45 seconds regardless of how fast you swim — the faster you go, the more rest you earn within that cycle.
Send-off = repeat swim time + rest. The repeat swim time is the distance divided by 100 multiplied by your pace per 100. For example, a 100 m repeat at 1:30 pace with 15 seconds rest gives a send-off of 1:45. Use the interval calculator above to compute this for any distance, pace, rest and rep count.
Rest is the main lever for changing the training stimulus of a set. Very short rest (5 to 10 seconds) keeps heart rate high and trains aerobic endurance and lactate tolerance. Moderate rest (15 to 20 seconds) suits threshold or CSS pace work. Long rest (30 to 60 seconds or more) allows near-full recovery and is used for race-pace and sprint repeats where speed, not fatigue, is the goal.
"On 1:30" means you start each repeat every 1 minute 30 seconds on the pace clock. If you finish a 100 m repeat in 1:20, you get 10 seconds rest before the next one leaves. If you finish in 1:35, you have already missed the send-off and the next repeat has started — a sign the set is too aggressive for your current pace. Send-offs reward faster swimming with more rest.