🏃 Half Marathon Pace Calculator

How to Use This Half Marathon Pace Calculator

A half marathon is 21.0975 kilometers (13.1094 miles), long enough that pacing mistakes compound brutally. Enter your goal finish time and the calculator returns the exact per-km and per-mile pace required, plus the intermediate splits to check against on course.

The Math

The half marathon distance is fixed at 21.0975 km, so pace is total seconds divided by 21.0975. A 2-hour goal is 7200 seconds: 7200 / 21.0975 = 341.27 seconds per km = 5:41. Per mile: 7200 / 13.1094 = 549.2 seconds = 9:09. Speed = 21.0975 / 2 = 10.55 km/h, or 6.55 mph. The math is exact; the judgment is in choosing a goal your fitness supports, which is where most half marathons are won or lost before the start.

Worked Example: Sub-2:00

Enter 120 minutes. Total = 7200 seconds. Pace per km = 7200 / 21.0975 = 341.27 s = 5:41. Pace per mile = 7200 / 13.1094 = 549.2 s = 9:09. Speed = 21.0975 / 2 = 10.55 km/h = 6.55 mph. Splits: 5K = 5:41 x 5 = 28:26; 10K = 56:52; 10 miles = 9:09 x 10 = 91:32, leaving 28:28 for the final 3.1 miles. Check each figure in the calculator. Notice the 10-mile split: if you reach 10 miles in 1:31:32, the goal is alive; if it is 1:35, adjust expectations rather than sprinting.

Race-Day Pacing Strategy

The half marathon punishes early aggression more than any shorter race. Run the first 5K at goal pace or a touch slower, never faster; the crowds and adrenaline make 15 seconds per km too fast feel easy, and it is not. Miles 4 through 10 are the business section: settle into rhythm, take every gel on schedule, and ignore surges from other runners. From mile 10, if you feel good, gradually press; the last 5K is where races are actually run. Walk breaks through water stations cost less time than you fear and save more energy than you expect, especially past mile 9. Heat and hills demand pace adjustments: add 10 to 20 seconds per mile on hot days rather than forcing the number.

Training Paces From Your Half Marathon Goal

Your goal pace anchors the training plan. Easy runs go 60 to 90 seconds per mile slower than half marathon pace. Tempo runs sit near your lactate threshold, roughly 15 to 25 seconds per mile faster than half marathon pace for most runners. Long runs start at easy pace and finish the last few miles at goal pace to practice running fast on tired legs. Interval sessions run at 5K to 10K pace, well faster than goal pace, to lift the ceiling. Twelve to sixteen weeks of this structure, with weekly mileage appropriate to your history, turns an honest goal pace into a finish-line reality.

Second worked example: 1:45 goal

Enter 105 minutes. Total = 6300 seconds. Pace per km = 6300 / 21.0975 = 298.66 s = 4:59 per km. Pace per mile = 6300 / 13.1094 = 480.56 s = 8:01 per mile. Speed = 21.0975 / 1.75 = 12.06 km/h = 7.49 mph. Splits: 5K = 4:59 x 5 = 24:53; 10K = 49:47; 10 miles = 8:01 x 10 = 80:06, leaving 24:54 for the final 3.1 miles. Compare with the sub-2:00 example: 15 minutes off the finish time costs 42 seconds per km of pace, 5:41 down to 4:59. That pace gap is roughly the difference between a 35-mile week and a 50-mile week of training, which tells you what the goal actually costs.

Half marathon benchmarks

Rough bands for adult runners: under 1:30 is strong club level, 1:30 to 1:45 is a good runner by any measure, 1:45 to 2:00 is solid recreational racing, 2:00 to 2:30 covers most first-timers and run-walkers, and finishing under the course cutoff, usually around 3:30 to 4:00, is the goal that matters most on debut. Women run about 10 to 12 percent slower than men at equal training, and age grading keeps comparisons fair past 40. Pick the band you are in, not the band you wish you were in, and set the goal from a recent 10K with a race predictor rather than from ambition.

Common half marathon mistakes

Mistake one is the 10K hero start. The first 5K feels easy at 15 seconds per km too fast, and the debt compounds into a death march from mile 10. Run the first 5K at goal pace or 5 seconds slow; you can always press later. Mistake two is skipping fuel. The body stores about 90 minutes of glycogen; a 2-hour half marathon runs out without gels, and the wall at mile 11 is not a fitness problem, it is a fueling problem. Take a gel every 30 to 40 minutes starting at 45 minutes. Mistake three is new anything on race day: shoes, gels, breakfast, shorts. Every novelty is a risk with zero upside; race in the boring gear you trained in.

For informational purposes only, not medical advice. Consult a healthcare professional.

The 13.1-Mile Fueling Plan

Pace gets you to mile 9; fueling gets you home. The standard guidance is 30 to 60 grams of carbohydrates per hour for efforts over 75 minutes, which means a sub-2:00 runner takes one or two gels during the race. Practice this in training: take a gel at mile 5 and mile 9 with water, never for the first time on race day. Skipping fuel to "run light" is the classic half marathon error; the bonk at mile 11 costs far more time than the 20 seconds lost grabbing a gel. Your stomach is trainable, but only if you train it.

Why the Second Half Is Slower: The Fade Rule

Most runners run the second half of a half marathon 2 to 5 percent slower than the first, even when aiming for even splits. Glycogen depletion, rising core temperature, and mental fatigue all compound after mile 8. The practical fix is banking discipline in reverse: run the first 3 miles 5 to 10 seconds per mile slower than goal pace, then let the pace come to you. A runner who goes out 15 seconds fast and fades 30 seconds slow loses a minute; the runner who starts controlled usually finishes within seconds of the plan. Check your splits at mile 5: if you are ahead of schedule and feeling great, hold it anyway. Mile 11 will test the decision.