What 5K Time Do I Need for a Sub-3-Hour Marathon?
A 21-minute 5K indicates you have the raw speed to target a sub-3-hour marathon.
How to use this tool
- Enter known race distance, finish time — minutes and finish time — seconds in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your marathon time and the full breakdown beneath it.
Check if your current 5K speed is fast enough to predict a sub-3-hour marathon finish.
Frequently asked questions
- How accurate is the Riegel formula?
- For distances within 2–3x of your reference race it is generally accurate within 5%. Accuracy decreases for very long extrapolations.
- What exponent should I use?
- The standard exponent is 1.06. Some calculators use 1.07 for ultra distances. This tool uses 1.06.