This calculator uses the Pete Riegel formula: T₂ = T₁ × (D₂/D₁)^1.07, where T₁ is your known time at distance D₁, and D₂ is the target distance. The exponent 1.07 is widely validated for swimming and accounts for the natural slowdown as distance increases. Different strokes use slightly adjusted exponents (1.065–1.075) for better accuracy.

1 yard = 0.9144 meters. A 100-yard swim is approximately 91.44 meters, making it about 8.6% shorter than 100 meters. Short course pools in the US are typically 25 yards (SCY), while international competitions use 25-meter or 50-meter pools (SCM/LCM). Our calculator handles both unit systems independently — enter your time in the system you train in for the most accurate predictions.

The Riegel formula has been validated across endurance sports and shows good accuracy for swimming distances between 50 and 1500 meters/yards. Typical prediction error is ±3–5%. Accuracy is best when predicting distances within 4× your benchmark (e.g., predicting 400m from a 100m time). For very short sprints (50m) or very long distances, predictions may vary more due to individual differences in endurance, technique, and pacing strategy.

Each stroke has a unique efficiency curve. Butterfly is the most demanding over distance, so its fatigue exponent is slightly higher (1.075). Breaststroke relies heavily on timing and glide, showing slightly less drop-off (1.065). Freestyle and Backstroke sit in the middle at 1.07. These subtle differences become noticeable at longer distances like 800m or 1500m.

Elite swimmers: 47–55 seconds (freestyle). Competitive age-group: 55–1:10. Fit recreational swimmers: 1:10–1:40. Beginners: 1:40–2:30+. Times vary significantly by age, gender, stroke, and training level. Use this calculator to set realistic goals for other distances based on your current 100m or 100yd benchmark.

The predictions assume pool conditions with consistent turns and no currents. Open water times are typically 5–15% slower due to navigation, waves, lack of walls for turns, and variable conditions. If you're training for an open water event, use the predicted time as a baseline and add 8–12% for a realistic open water estimate.

Critical Swim Speed (CSS) is the theoretical maximum pace a swimmer can sustain continuously without exhaustion — essentially your lactate threshold pace. CSS is typically calculated from 400m and 200m time trials. While this calculator uses 100m as the benchmark, the predicted 400m pace provides a reasonable approximation of your CSS pace, which is excellent for setting training intensities.