Swimming Glossary

Swimming has its own language. The sport borrows terms from physiology, track athletics, rowing and even golf, and then layers on pool-specific vocabulary that does not appear anywhere else. This glossary defines every metric and term used across the calculators and resource pages on this site, in plain English with a link to the relevant calculator where one exists. Terms are grouped roughly by theme: physiology and pacing first, then technique, then pool and training vocabulary.

Pacing & physiology

CSS (Critical Swim Speed): The fastest pace you can sustain without rapid fatigue — the swimming equivalent of lactate threshold pace. It is calculated from two maximal time trials (200 m and 400 m) and expressed as a speed in m/s or a pace per 100 m. CSS is the anchor for all pace-based training zones. Calculate your CSS →

Threshold: The exercise intensity at which lactate begins to accumulate in the blood faster than the body can clear it — the boundary between sustainable aerobic effort and rapidly fatiguing anaerobic effort. In swimming, threshold pace is practically synonymous with CSS pace, and training at threshold is what raises it over time.

Pace: Time taken to cover a fixed reference distance, almost always expressed per 100 m or per 100 yd. Pace is the standard way to compare efforts across different distances: a 1500 m swim and a 400 m swim can both be reduced to a per-100 pace and compared directly. Pace calculator →

Speed: Distance covered per unit of time, usually metres per second (m/s) or kilometres per hour (km/h). Speed is the inverse of pace and is the more natural unit for physics-based analysis and for comparing swimming with other sports. Speed calculator →

Split: The time for a single segment of a longer swim — for example, each 50 m of a 200 m race, or each 100 m of a 400 m set. Splits reveal pacing strategy: even splits (each segment the same time) are usually the fastest way to swim distances of 200 m and up. Split calculator →

Negative split: A swim in which the second half is faster than the first — for example, a 400 m swum as 2:10 then 2:05. Negative splitting teaches pace discipline and is a common race strategy for distances from 200 m up, because it avoids the early anaerobic debt that causes a hard-fade finish.

Even split: A swim in which every segment is completed in the same time. Even splitting is generally the most energy-efficient way to pace any distance from 200 m upwards and is what most target-time calculations assume.

Pace clock: The large analogue or digital clock on the pool wall used to time intervals and send-offs. Reading a pace clock — working out when to leave on a given send-off time — is a core pool skill. Most pace clocks have a 60-second face; send-offs are spoken as “on the 40” (leave when the second hand hits 40) or “on the top” (leave on the minute).

Training & intervals

Interval: A training set made up of repeated swims on a fixed time cycle, where each repeat starts at a set time regardless of how long the swim took. The interval creates rest automatically: swim faster, get more rest; swim slower, get less. Intervals are the structure of almost all swim training. Interval calculator →

Send-off: The clock time at which you leave on each repeat of an interval set. A send-off of 1:40 means you start every rep 1:40 after the previous one, so if you swim 100 m in 1:30 you get 10 s rest. The send-off is the key variable that controls the difficulty of a set.

Kick set: A set swum using the legs only, usually with a kickboard or with the arms held still. Kick sets develop leg propulsion and body position independently of the arm stroke, and they are a staple warm-up and technique component.

Pull set: A set swum using the arms only, with a pull buoy between the legs to keep them still and high. Pull sets build upper-body strength and let you focus on arm technique and catch without the legs contributing.

Drill: A technique-focused exercise that isolates part of the stroke — for example, single-arm freestyle, fist drill, or catch-up. Drills are swum slowly and deliberately to groove a movement pattern, not for fitness.

Taper: The reduction in training volume (and often intensity) in the one to three weeks before a target race, intended to shed fatigue while maintaining fitness so that you arrive at the start fully recovered and fast. A well-judged taper can shave seconds off race times; a poor one leaves you either stale or under-trained.

Technique metrics

SWOLF: A swimming efficiency score: the sum of your time in seconds and your stroke count over a fixed distance (usually 25 m or 50 m). The name comes from “swim + golf,” and like golf a lower score is better. Tracking SWOLF over time at a fixed distance is a direct, objective measure of technique improvement. SWOLF calculator →

DPS (Distance Per Stroke): The distance you travel on a single stroke, in metres per stroke. DPS is the complement of stroke rate: DPS × stroke rate = speed. Improving DPS — going further on each stroke — is the primary technique goal for most adult swimmers. Stroke length calculator →

SPL (Strokes Per Length): The number of strokes you take to cover one length of the pool. A lower SPL generally indicates a longer, more efficient stroke, though a very low SPL achieved by over-gliding can be slow. SPL is the raw input to DPS — divide the length distance by SPL to get DPS.

Stroke rate: The cadence of your swimming, measured in strokes per minute (SPM). It is the swimming equivalent of cycling cadence or running stride rate. Most efficient freestylers sit between 30 and 50 SPM in steady swimming; sprinters can exceed 70 SPM. Stroke rate calculator →

Catch: The moment at the front of the stroke when your hand and forearm grip the water and begin to pull. A good catch sets up the whole propulsive phase; a poor catch — slipping through the water with a dropped elbow — wastes the stroke. Most technique drills target the catch.

Pull: The propulsive phase of the arm stroke, from the catch through to the push past the hip. This is where the majority of freestyle, backstroke and butterfly propulsion is generated, and it is the phase most affected by strength and by the quality of the catch.

Recovery: The non-propulsive phase of the stroke when the arm returns over the water to the front. In freestyle and backstroke the recovery should be relaxed and high-elbow; a tense or low recovery wastes energy and can upset body rotation.

Rotation: The rolling of the body around its long axis during each stroke cycle. Good rotation (roughly 30–45° from flat) lengthens the reach, helps the catch and engages the larger core and back muscles in the pull. Swimming flat is a common efficiency fault.

Bilateral breathing: Breathing on both sides, typically every three strokes (alternating left and right). Bilateral breathing keeps the stroke balanced and the rotation symmetrical, which helps avoid a lopsided stroke and is especially useful in open water for sighting and adapting to waves.

Sighting: The technique of lifting the eyes just above the surface to look forward while swimming in open water, in order to navigate without a pool wall or lane line. Efficient sighting — a brief, low “alligator eyes” lift integrated into the stroke — is essential for swimming straight and short in races.

Drafting: Swimming directly behind or just off the hip of another swimmer to benefit from reduced water resistance. Drafting can save 10–20% of energy in open water and is a key racing tactic; it also slightly reduces the drafter's drag on the leader.

Strokes & events

Freestyle: The fastest and most common stroke, swum with an alternating arm windmill and a flutter kick, with the face down and the body rotating side to side. In competition “freestyle” means any stroke is allowed, but front crawl is universally used because it is fastest.

Breaststroke: A simultaneous, symmetrical stroke: both arms sweep inward together while both legs perform a frog kick. It is the slowest of the four competitive strokes and the most technically demanding in terms of timing, but it is also the easiest to swim gently and is often taught first.

Backstroke: The only stroke swum on the back, with an alternating arm windmill and a flutter kick. Backstroke is started in the water with a push-off and is the only stroke in which you cannot see where you are going, which makes it impractical in open water.

Butterfly: A simultaneous, symmetrical stroke: both arms recover over the water together while both legs perform a dolphin kick. It is the most physically demanding stroke and burns the most calories per minute, which is why its MET value is far higher than the others.

IM (Individual Medley): A race in which one swimmer swims all four strokes in a fixed order: butterfly, backstroke, breaststroke, freestyle. The order differs in a medley relay (backstroke, breaststroke, butterfly, freestyle). IM demands versatility and rewards swimmers with no single weak stroke.

Pool & units

Length: One way across the pool, from one wall to the other. In a 25 m pool one length is 25 m. Counting lengths is the simplest way to track distance without electronics, and “length” is the unambiguous unit to use when reporting how far you swam. Pool length calculator →

Lap: There and back — two lengths. In a 25 m pool one lap is 50 m. The word is frequently misused to mean a single one-way trip, which is why this site always counts lengths instead; see the units guide for the full explanation.

SCM (Short Course Metres): A 25 m pool, the standard short-course format used everywhere outside the United States. Short course has more turns per race than long course, and the turns contribute significantly to overall time.

SCY (Short Course Yards): A 25 yd (22.86 m) pool, the standard short-course format in the United States. Because a yard is shorter than a metre, SCY times are not directly comparable to SCM times and are kept as separate records.

LCM (Long Course Metres): A 50 m pool, the Olympic and world-championship format. Long course has half as many turns as short course over the same distance, so pure swimming speed matters more and times are generally slower than in short course.

MET (Metabolic Equivalent of Task): A number expressing the energy cost of an activity relative to resting. One MET is the energy you burn at rest; swimming freestyle at moderate effort is about 8.3 METs, meaning you burn 8.3× your resting rate. MET values are the basis of calorie calculations. Calorie calculator →

For the formulas behind these metrics, see the swimming formulas page; for how to turn your CSS into training intensities, see the training zones guide.

Pacing & physiology

Training & intervals

Technique metrics

Strokes & events

Pool & units

Use these formulas in our calculators