Health⏱ 6 min read
How to Calculate Your Pace for a Marathon or Half Marathon
Race pace strategy can be the difference between a PB and a miserable last 10km. Here's how to calculate your target pace, build a pacing plan, and avoid the most common race-day mistake.
Finishing a marathon strong requires starting conservatively. The maths of race pacing is simple — the psychology of sticking to it on race day is harder. Here's how to calculate the numbers and understand why they matter.
Basic Pace Calculations
Pace (min/km) = Total time (minutes) / Distance (km)
Target finish: 4 hours for marathon (42.195km)
Target time: 240 minutes
Pace = 240 / 42.195 = 5:41 per km (5 min 41 sec per km)
Or per mile: 4:00:00 marathon = 9:09/mile
For half marathon (21.0975km):
2:00:00 = 120 / 21.0975 = 5:41/km (same per-km pace)
1:45:00 = 105 / 21.0975 = 4:58/km
Common Target Times and Required Paces
Target TimePer kmPer mileHalf split
3:30 marathon4:58/km8:00/mi1:45
4:00 marathon5:41/km9:09/mi2:00
4:30 marathon6:23/km10:17/mi2:15
5:00 marathon7:06/km11:26/mi2:30
1:45 half4:58/km8:00/mi—
2:00 half5:41/km9:09/mi—
2:15 half6:23/km10:17/mi—
Negative Splitting: The Evidence-Based Strategy
A negative split means running the second half faster than the first. Every marathon world record has been set with a negative or even split. Starting 5–10 seconds per km slower than target pace in the first half and building feels counterintuitive but consistently produces better results.
Example: Targeting 4:00 marathon (5:41/km average)
Positive split (too common): first half at 5:30/km, second half 5:55/km
Result: bonk at km 32, stagger to 4:10+
Negative split: first half at 5:48/km, second half at 5:34/km
Result: comfortable first half, strong finish, 4:00 achieved
10-second difference in early pace = 7-minute difference in finish time
The Wall: What Happens at km 32
Glycogen depletion — "hitting the wall" — typically occurs around 30–35km when liver and muscle glycogen stores are exhausted. Prevention:
- Carb loading: 2–3 days before, increase carbohydrate intake to 8–10g per kg body weight per day
- Race nutrition: 30–60g of carbohydrate per hour during the race (gels, sports drinks)
- Conservative early pace: Higher intensity burns glycogen faster; slower early pace extends stores
Estimating Marathon Time from Shorter Race Times
Riegel formula (most widely used):
T2 = T1 x (D2/D1)^1.06
T1 = known race time, D1 = known distance
T2 = predicted time, D2 = new distance
Example: ran half marathon in 1:55:00 (6,900 seconds)
Marathon prediction: 6,900 x (42.195/21.0975)^1.06
= 6,900 x (2.0)^1.06
= 6,900 x 2.084 = 14,382 seconds = 3:59:42
Note: Riegel formula is optimistic for those without marathon-specific training.