You probably have a goal time in your head that came from a friend, a bucket list, or a round number. Your recent races and training say something different, and they’re usually right. Start there, and your last 5 km gets a lot easier.
This guide gives you a three-step way to pick a half marathon goal pace you can hold. It also shows what 423,496 real race results say happens when runners ignore it.
Why a wish time is the wrong starting point
In a study of 423,496 results from the Gothenburg Half Marathon, only 9.8% of runners ran an even or negative split (Johansson et al., 2023). The average runner slowed in every 5 km block until 20 km. In short: most people start faster than they can finish.
A runner finishing near 2:00 lost just over 4 minutes in the second half. Runners under 1:30 lost about 1:30. Men finishing over 2:30 had a severe slowdown more than 30% of the time.
A wish time is like a budget you wrote before you saw your paycheck. Your training is the paycheck.
Step 1: Start with a recent race
Use a 5K, 10K, or tune-up from the last 6 to 8 weeks. Pick the one run on a flat course in decent weather. A hot, hilly 10K will make you look slower than you are.
The Riegel formula turns one race time into another distance:
T2 = T1 * (D2 / D1) ^ 1.06
In plain terms: when distance doubles, time goes up about 2.08x, not 2x. Run twice as far and you don’t hold the same pace.
Step 2: Convert, and know where it breaks
These multipliers are our arithmetic from the formula, not published numbers: 4.600 for a 5K to half, and 2.206 for a 10K to half. The table below is calculated from them.
| Recent race | Riegel half time | Pace per km | Pace per mile |
|---|---|---|---|
| 5K in 20:00 | 1:32:00 | 4:22 | 7:01 |
| 5K in 25:00 | 1:55:00 | 5:27 | 8:46 |
| 5K in 30:00 | 2:18:00 | 6:32 | 10:32 |
| 10K in 40:00 | 1:28:15 | 4:11 | 6:44 |
| 10K in 45:00 | 1:39:17 | 4:42 | 7:34 |
| 10K in 50:00 | 1:50:19 | 5:14 | 8:25 |
| 10K in 55:00 | 2:01:21 | 5:45 | 9:15 |
| 10K in 60:00 | 2:12:23 | 6:16 | 10:06 |
These are estimates. Treat them as best case if you run under about 30 km a week, your longest run is under 16 km, or you’ve done no threshold work.
A cross-check helps. Daniels’ VDOT 40 lists a 24:06 5K, a 50:00 10K, and a 1:50:52 half. Riegel on that 24:06 5K lands on the same 1:50:52. On the 10K they differ by about 30 seconds. Close enough to trust.
Why Riegel works for the half but not the marathon
Vickers and Vertosick studied 2,303 recreational runners. Riegel worked well up to the half. At the marathon it was at least 10 minutes too fast for half of them (BMC Sports Sci Med Rehabil, 2016). So the half is the distance where the formula is most trustworthy.
Don’t flip it around. A half time won’t safely predict a marathon, and the same caution applies to a 70.3 run split.
Step 3: Check the number against your training
A calculator only knows one race. Your training knows more. Ask three questions.
- Volume. Are you running 30+ km most weeks? Less than that, and the formula flatters you.
- Long run. Have you done 16 km or more, with the last stretch near goal effort?
- Tempo work. Have you held a steady, hard effort for 20 to 30 minutes? Many coaches expect threshold pace to be faster than half marathon pace for a runner racing about two hours. That’s practical coaching guidance, not a published ratio.
If you answer yes to all three, trust the calculator. If you answer no to one, add 2-3% to the time. If you answer no to two, use the conservative number in the planner below.
A case study
Dana is 41 and 14 weeks from her first sub-2:00 attempt. She’s self-coached, runs about 38 km a week, and has a 25:00 5K from August. Riegel says 1:55:00, or 5:27 per km. Her wish was 1:50, which needs 5:13 per km.
Her longest run was 14 km. She had done one tempo session of 15 minutes. So her 1:55 was a best case, not a plan.
Dana made 1:58 her target (5:36 per km). She built the long run to 18 km and the tempo to 25 minutes. In week 12 she ran a 10K in 53:40, which Riegel turns into about 1:58:25. The target held. She raced it with an even first 5 km and finished in 1:57:30.
Her goal got slower on paper. Her race got faster in real life.
Half marathon pacing: what to run in the first 5 km
A good coaching rule of thumb: run the first 5 km 3-5 seconds per km slower than goal pace. Then settle onto goal pace. Then, if you still feel strong at 16 km, push.
Think of your legs as a phone battery. A fast start drains 10% in the first mile and leaves you hunting for a charger at 18 km. Steady is faster than fast-then-slow.
Elite runners often run even or slightly negative splits. A 2025 review of marathon pacing notes that recreational runners tend to slow, and that slowing tracks with worse results and higher effort. For the half, an even split is the realistic target. A true negative split is a bonus. For a deeper look, see the pacing math behind negative splits.
Heat: plan on minutes, not seconds
In the Gothenburg data, the warmest year (25 C) was 9:18 slower for men than the coolest (13.6 C). Women were 7:36 slower. Men’s severe slowdown rate went from 7.1% to 16.8%. These are year-level averages across 10 years, not a rule for one runner.
Marathon data points the same way. The top finishers ran 1.7%, 2.5%, 3.3%, and 4.5% off the course record from the coldest to the warmest temperature quartile (Ely et al., 2007). Slower runners lost more than the fastest. That’s marathon data, so treat it as a guide for the half.
Heat lies about your pace. Your effort doesn’t.
Hills: pace by effort
Running uphill costs much more energy. Downhill gives back less than you’d think. Minetti’s work tested extreme grades, so it doesn’t give a pace per 1% of road grade (J Appl Physiol, 2002). The practical point holds: on a rolling course, flat-pace averaging fails. Let pace slip on climbs, hold effort, and don’t chase it back on the descent.
Build an A, B, and C goal
| Goal | Pace vs. calculator | Use it when |
|---|---|---|
| A (stretch) | Calculator time | Volume, long run, and tempo all check out. Cool, calm morning. |
| B (target) | About 2-3% slower | One check is shaky, or it’s mild warm. |
| C (protect) | About 5% slower or more | Hot day, poor sleep, or a rough build. |
Decide at 5 km, not at the start line. If the first 5 km feels easy at B pace, you can hold it or move toward A. If it feels hard, drop to C and finish well.
Rest matters too. Your taper shapes how fresh your legs feel on race morning, so read how long to taper for a half marathon. If your build got derailed, missing two weeks of half marathon training doesn’t have to kill your goal, but it can change which pace is realistic.
Keeping the number honest
A goal pace set 20 weeks out is a guess. Training changes it. AthleteOS builds your training plan from your real training data. It adapts when you miss sessions, sleep badly, travel, or get sick. It also explains each change, so you know why your training shifted.