A half marathon is 13.1094 miles (21.0975 km) — exactly half of a full marathon — and it is the distance where most recreational runners discover whether they actually love road racing. The single number that decides whether your race feels controlled or disastrous is the pace you hold from mile 1. This half marathon pace calculator takes your target finish time and converts it into a clean per-mile and per-kilometer pace, plus the 5K and 10K split times you should be hitting on course, and a Riegel projection of what that half effort might translate to over a full 26.2 miles.
Whether you are chasing a sub-1:45 debut, trying to break two hours comfortably, or simply aiming to finish upright after 13.1 miles, the math is the same: divide your goal time by the distance and you have the rhythm to lock in. Enter your target hours, minutes, and seconds below and watch every figure update instantly.
How to Use
- Enter your Target Hours — the hour portion of the finish time you want. A common first goal is 2 hours; a competitive age-group goal is 1 hour 30 minutes.
- Add your Target Minutes — for example, if you want to finish in 1:45, enter 45 here.
- Add your Target Seconds — optional, but useful if you are chasing a specific badge like sub-1:45:00.
- Read the result card: your pace in minutes per mile and per kilometer, the exact 5K and 10K splits you need to stay on track, and a projected marathon time using the Riegel Formula.
- Use the splits to build a race-day plan: pin the 5K and 10K targets to your hydration belt and check your watch at each mat.
A 1:45 half marathon requires an average of 8 minutes 1 second per mile (about 4:59 per kilometer). That is the rhythm you should settle into in the first mile rather than chasing the fast starters around you.
The Pace Formula
Pace is just total time divided by distance. The engine converts your hours, minutes, and seconds into a single second count, then divides by the fixed half-marathon distance:
paceSecPerMile = (hours × 3600 + minutes × 60 + seconds) ÷ 13.1094
To convert to kilometers, divide by 1.609344 — the exact conversion factor that defines a mile. The 5K (5 km) and 10K (10 km) splits are that per-kilometer pace multiplied by the split distance. This assumes even pacing, which is the single biggest predictor of a strong half marathon result.
Worked example: targeting a 1:45:00 finish means 6,300 seconds over 21.0975 kilometers, or 298.6 seconds per kilometer — 4:59 per km and 8:01 per mile. At that pace you hit the 5K mat around 24:53 and the 10K mat near 49:46, then close the final 3.1 km on almost exactly the same rhythm.
Predicting Your Marathon with Riegel
The Riegel Formula, published by Peter Riegel in 1981, estimates race time at a new distance from a known performance:
T2 = T1 × (D2 ÷ D1) ^ 1.06
Plugging a 1:45:00 half (21.0975 km) into a marathon (42.195 km) gives roughly 3:38:55 — a useful starting point for choosing a marathon goal, not a guarantee. Real marathon performances usually land 2–5% slower than the projection because of fatigue, fueling, and the sheer length of the event. Use the number as a planning band, not a race-day promise.
Why Even Pacing Matters
Recreational runners who start the first mile faster than goal pace almost always pay for it in the final 3.1 miles. Analyses of large race fields consistently show that runners who open more than 5% faster than their target pace are far more likely to slow down dramatically after the 10K mark. The fix is boring but effective: run the pace this calculator gives you from the opening kilometer, even if the crowd is screaming and the runner next to you looks smooth.
Most half-marathon training plans build a weekly long run that peaks at 10–12 miles. Use this tool to translate your goal pace into long-run effort: if half-marathon pace is 8:01/mile, your easy long-run pace should be 9:00–9:45/mile, and your tempo sessions should flirt with 7:30–7:45/mile. If you cannot hold those training bands comfortably in the final six weeks, adjust your goal finish time rather than risking a blowup on course.
Race entry fees for a major half marathon run $60–$150, and travel plus a hotel on top of that can push a destination race to $500–$1,000. Treat the pace plan as seriously as the registration cost.
Examples
Example 1 — A 1:45 goal. You want to break 1:45. Set hours = 1, minutes = 45, seconds = 0. The calculator returns 8:01 per mile and 4:59 per km, with a 5K split near 24:53 and a 10K split near 49:46. The projected marathon is roughly 3:39. Tape the splits to your water bottle and run the first 5K dead on; the final 5K will feel hard but survivable.
Example 2 — A 2:00 first half. A first-timer aiming to finish under two hours sets hours = 2, minutes = 0, seconds = 0. That is 9:09 per mile (5:41 per km), with a 5K split around 28:26 and a 10K split around 56:53. At that intensity you should already be logging weekly mileage of 25–35 miles, and your fueling — roughly 30–60 grams of carbohydrates per hour, or about one gel every 45 minutes — should be rehearsed on long runs, not invented on race morning.
Example 3 — A sub-1:35 attempt. Ambitious runners targeting 1:35:00 set hours = 1, minutes = 35, seconds = 0. That is 7:15 per mile (4:30 per km) for 13.1 miles. At that pace you should be comfortable holding tempo near 6:45/mile in training, and you should have already finished a tune-up 10K within about two minutes of this projected effort. If your 10K PR is slower than that, build more aerobic fitness before committing to the time goal.
FAQ
What pace do I need for a 1:45 half marathon?
A 1:45 half marathon requires an average of 8 minutes 1 second per mile, or 4 minutes 59 seconds per kilometer. At that pace you pass 5K around 24:53 and 10K around 49:46, and the Riegel projection suggests a marathon in the low 3:30s if you extend your training appropriately.
What pace is a sub-2:00 half marathon?
To finish under 2 hours you need to average 9 minutes 9 seconds per mile, or 5 minutes 41 seconds per kilometer. That is a very approachable goal for a runner who can comfortably cover 30–35 miles per week and has completed a handful of 10K races in the 55–65 minute range.
How do I convert minutes per mile to minutes per kilometer?
Divide the per-mile pace by 1.609344. For example, 8:01 per mile becomes 4:59 per km. Most running watches display both units, but if yours only shows one, use this factor to cross-check against the kilometer signs on an international course.
What should my pace be for the first half vs the second half?
The safest plan is even pacing — the same per-mile pace for the entire race. Some experienced runners can negative-split by 1–2 minutes, but age-group runners who open more than 5% faster than goal pace almost always fade in the final 3 miles. Use the splits this calculator gives you and hit the 10K mat exactly on time.
How accurate is the Riegel marathon prediction?
The Riegel Formula is a planning estimate, not a guarantee. Most runners finish their first marathon 2–5% slower than the projection because of fatigue, fueling mistakes, heat, and the sheer length of the event. Use the predicted time here as a starting point for choosing a goal, then adjust after a few long runs at marathon-specific effort.
What is a good half marathon pace for a beginner?
Most first-time half marathoners finish between 2:00 and 2:40, which is 9:09–12:13 per mile. If you can comfortably finish a 10K at your goal half-marathon pace in training, you are in a realistic range. Do not try to race at a pace you have not already practiced on long runs.
How much does elevation change my pace?
This calculator assumes a flat course. Each 100 feet of climbing typically adds about 2–3 seconds per mile to your realistic pace, and a net downhill course can subtract a similar amount. If your race has more than 300 feet of elevation gain, add 3–8 minutes to your expected finish time.
Disclaimer
This half marathon pace calculator is provided for informational purposes only and does not constitute coaching, medical, or training advice, and is not medical advice. Pace figures are based on the simple average formula and the Riegel projection, and they assume flat terrain and even pacing; real performances vary with elevation, weather, wind, fueling, fitness, and race-day conditions. If you are new to distance running, managing a health condition, recovering from injury, or increasing mileage significantly, consult a qualified coach or healthcare provider before beginning a training plan.