Half Marathon Time Calculator: Pace & Finish

A half marathon time calculator estimates your finish time from your average pace, or works backward to find the pace you need to hit a target time. You enter either your planned pace or your goal finish time and distance (always 21.0975 km / 13.1094 miles), and it returns the missing value, along with your average speed. It is a simple division problem, but the calculator removes the unit-conversion errors that trip up many runners.

Formula

The core relationship is: pace = time ÷ distance, and equivalently time = pace × distance. In symbols:

• t = finish time in minutes
• d = distance in kilometers (for a half marathon, d = 21.0975 km)
• pace = minutes per kilometer (min/km)
• speed = kilometers per hour (km/h)

The calculator uses two formulas:

• pace = t ÷ d
• speed = d ÷ (t ÷ 60) — the ÷60 converts minutes to hours.

If you prefer miles, use pace = t ÷ (d × 1.609344) to get minutes per mile, and speed = (d × 1.609344) ÷ (t ÷ 60) for mph. The calculator above works in metric by default, but you can convert your inputs manually.

Worked Example

Using the default values: t = 50 minutes, d = 10 km.

1. Pace = 50 ÷ 10 = 5.00 min/km.
2. Speed = 10 ÷ (50 ÷ 60) = 10 ÷ 0.8333 = 12.00 km/h.

So a 50-minute 10 km run is a 5:00 min/km pace, or 12 km/h. For a half marathon (21.0975 km), if you run 5:00 min/km, your finish time would be 21.0975 × 5 = 105.49 minutes, or 1:45:29.

How to Use the Calculator for Half Marathon Training

There are two common ways to use it:

• Predict your finish time: Enter your recent race pace (e.g., a 10 km pace) and the distance 21.0975 km. The calculator gives you the finish time if you hold that exact pace. Be aware that most runners slow down slightly in the second half, so add 5–10 seconds per km for a realistic goal.
• Find your target pace: Enter your goal finish time (e.g., 1:45 = 105 minutes) and the half marathon distance. The result is the pace you must hold (e.g., 4:58 min/km). This is your “even splitâ€� pace.

For the calculator above, remember to enter time in minutes, not hours. For 1:45:30, enter 105.5 minutes.

Common Mistakes and How to Avoid Them

• Mixing units: The formula works with any consistent units, but if you enter miles while the calculator expects kilometers, your pace will be off by a factor of 1.609. Always check the unit label.
• Forgetting the half marathon distance: It is 21.0975 km, not 21 km. The extra 0.0975 km adds about 30 seconds for a 5:00 min/km runner.
• Using pace from a shorter race: A 5 km pace is typically 15–30 sec/km faster than a half marathon pace. Use a recent half marathon or a tempo run pace instead.
• Rounding too early: If you round 21.0975 to 21, you lose about 30 seconds over the race. Keep full precision until the end.

Limits of the Formula

The pace–time–distance relationship is pure math, so it is exact. However, it assumes you run at a perfectly constant pace, which no one does. Real factors include:

• Course elevation: Uphill sections slow you down more than downhill sections speed you up.
• Weather: Heat, humidity, and wind affect pace significantly.
• Fueling and hydration: Poor nutrition can cause a late-race slowdown.
• Training and fatigue: Your body cannot hold a 10 km pace for 21 km.

Use the calculator as a planning tool, not a guarantee. Most pacing strategies aim for a slight positive split (first half a few seconds slower) or even splits.

Alternatives to a Simple Pace Calculator

If you want a more personalized prediction, consider these advanced methods:

• Riegel formula: Predicts finish time from a recent race using an exponent (typically 1.06). For example, a 10 km time of 50:00 predicts a half marathon of 50:00 × (21.0975/10)^1.06 ≈ 1:50:30.
• Jack Daniels’ VDOT: Uses a table to convert race times to a training paces. This gives not just a goal pace but also easy and interval paces.
• Heart-rate-based pacing: Uses your lactate threshold heart rate to set pace, which is more accurate for hilly courses.

These methods require more data, but they account for individual fitness better than a simple division.

Frequently Asked Questions

How do I convert my pace from min/km to min/mile?

Multiply your min/km pace by 1.609344. For example, 5:00 min/km becomes 5 × 1.609344 = 8:03 min/mile (since 0.609344 minutes is about 37 seconds). Conversely, divide min/mile by 1.609344 to get min/km.

What is a good half marathon time for a beginner?

For a first-time runner, finishing in under 2:30 (about 7:06 min/km) is a common goal. A more experienced beginner might aim for 2:00 (5:41 min/km). The “good� time depends on age, gender, and training background.

How much slower should my half marathon pace be than my 10 km pace?

Typically, half marathon pace is about 10–20 seconds per km slower than 10 km race pace. For a 50-minute 10 km runner (5:00 min/km), a half marathon pace of 5:10–5:20 min/km is reasonable.

Can I use this calculator for a full marathon?

Yes, but change the distance to 42.195 km. The math is the same. However, the full marathon involves more fatigue and fueling, so the simple formula may overestimate your ability to hold pace.

Why is the half marathon distance 21.0975 km?

It is exactly half of a full marathon (42.195 km). The full marathon distance was standardized in 1921 by the IAAF, and the half marathon naturally became half of that.

How do I calculate my finish time if I run a negative split?

If you plan to run the first half slower and the second half faster, you can’t use a single pace. Instead, calculate the time for each half separately using the same formula, then add them. For example, 10.5 km at 5:10 min/km plus 10.5 km at 4:50 min/km gives 54:15 + 50:45 = 1:45:00.

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in t | Finish time | min | 50 | 0
in d | Distance | km | 10 | 0
out pace | Average pace | min/km | t / d | 2
out speed | Average speed | km/h | d / (t / 60) | 2