Overview
Endurance athletes rely on a handful of standard formulas to translate a known result into a training or race-day target — predicting a marathon finish time from a recent 10K, pacing a race using checkpoint splits, or converting a 20-minute cycling test into a full set of power training zones. This guide covers four calculators built around these exact conversions: race time prediction, marathon pacing, swim pace, and cycling FTP.
These tools are built for recreational and competitive endurance athletes planning a race, coaches setting training paces for athletes, and anyone curious about the math connecting a training test result to a race-day strategy.
Step 1: Predicting Race Times Across Distances (Riegel's Formula)
Riegel's formula predicts finish time at a new distance from a known result at a different distance:
T2 = T1 × (D2 ÷ D1)^1.06
The exponent 1.06 (rather than exactly 1.0) reflects that pace naturally slows as distance increases — you can't sustain your 5K pace for a marathon, so the formula scales time up slightly faster than distance alone would suggest. This makes it useful for setting a realistic goal time at an upcoming race distance based on a recent result at a different distance.
The Race Time Predictor Calculator takes your known time and distance, plus a target distance, and returns the predicted finish time instantly.
Worked example: A runner who completes a 10K in 50 minutes wants to predict their marathon (42.195 km) time: T2 = 50 × (42.195/10)^1.06 ≈ 50 × 4.22^1.06 ≈ 50 × 4.59 ≈ 229.5 minutes, or about 3 hours 50 minutes.
Step 2: Marathon Pacing and Checkpoint Splits
Once you have a target marathon finish time, the next question is what pace to hold — and, critically, what your split times should look like at common checkpoints (10K, half marathon, 30K) so you can check whether you're on pace mid-race rather than discovering a problem only at the finish line.
Required Pace = Target Time ÷ Total Distance
The Marathon Pace Calculator computes your required per-km or per-mile pace from a target finish time, then generates the corresponding split times at each standard checkpoint distance.
Worked example: For a target finish time of 4:00:00, required pace is 9:09 per mile — with splits of roughly 1:31:04 at 10K, 1:59:00 at the half marathon, and 2:50:31 at 30K, assuming even pacing throughout.
Step 3: Swim Pace Conventions
Swimming pace is conventionally expressed per 100 meters or per 100 yards, rather than per mile like running, because pool training sets and competition distances are structured around 25m/25y or 50m/50y pool lengths. Converting a total swim distance and time into this standard per-100 figure lets you compare your pace against training plans and previous results on a consistent basis.
Pace per 100 = (Time ÷ Total Distance) × 100
The Swim Pace Calculator takes your total distance (with a pool-length unit selection for meters or yards) and time, returning pace per 100 in min:sec format.
Worked example: Swimming 1,500 meters in 25 minutes gives a pace of (25 ÷ 1500) × 100 = 1.667 minutes per 100m, or 1:40 per 100m.
Step 4: Cycling FTP and Power Training Zones
Functional Threshold Power (FTP) is the highest average power (in watts) a cyclist can sustain for approximately one hour. Since a true 60-minute maximal test is difficult to pace and mentally taxing, FTP is commonly estimated from a shorter, more practical 20-minute maximal effort:
FTP ≈ 20-Minute Average Power × 0.95
Once FTP is known, training intensity is typically divided into the 7 Coggan power zones — Active Recovery, Endurance, Tempo, Lactate Threshold, VO2 Max, Anaerobic Capacity, and Neuromuscular Power — each defined as a percentage range of FTP, giving a structured way to target specific physiological adaptations in training.
The Cycling FTP Calculator takes your 20-minute test average power and returns both the estimated FTP and the full 7-zone breakdown in watts.
Worked example: A 20-minute test average of 250 watts gives FTP = 250 × 0.95 = 237.5 watts — with Zone 2 (Endurance) typically spanning roughly 56-75% of FTP, or about 133-178 watts for this rider.
Putting It Together: From Test Result to Race Day
These four calculators cover different phases of endurance training, but they're most useful chained together. A cyclist might use a 20-minute FTP test early in a training block to set accurate power zones, then train predominantly in the Endurance and Tempo zones for several weeks to build aerobic base. A runner preparing for a marathon might use a recent 10K or half-marathon result with the Race Time Predictor to set a realistic goal time, then feed that goal time into the Marathon Pace Calculator to get concrete checkpoint splits for race day — giving a clear go/no-go signal at 10K, halfway, and 30K rather than only finding out at the finish line whether the pace was sustainable.
Swimmers training for open-water or triathlon events often use per-100 pace targets from recent pool sessions to structure interval sets — for example, repeating 400m efforts at goal race pace with short rest, then checking the actual per-100 split against the calculator's target to gauge fitness progress week over week.
The common thread across all four tools is the same principle: use a recent, honest test result (not a hoped-for one) as the input, since every one of these formulas is only as accurate as the effort and pacing of the test or race it's built from. A 20-minute FTP test ridden too conservatively understates true FTP just as a 5K time trial run well under maximal effort will understate true race fitness — both will throw off every downstream calculation in this guide.
Key Terms
- FTP (Cycling) — Functional Threshold Power, the highest power output sustainable for about one hour
- Riegel's Formula — a race-time prediction formula that scales time by distance ratio raised to the power 1.06
- VO2 Max — the maximum rate of oxygen consumption during intense exercise, an aerobic fitness ceiling
- Checkpoint Split — the target time at an intermediate distance within a longer race, used to verify pacing
- Coggan Power Zones — a 7-tier training intensity framework expressed as percentages of FTP
- Negative Split — running or riding the second half of an effort faster than the first half