Boat Fuel Range Calculator
Boat run time and cruise range with a 10% reserve.
What it does
Boat range starts with usable fuel, not tank size. This page holds back 10% as reserve, divides the rest by cruise burn for hours, and multiplies by cruise speed for distance. Both unit sets give the same trip shape.
The default is 30 gal at 6 gph and 25 mph. Usable fuel is 27 gal, time is 4.5 h, and range is 112.5 mi. Metric defaults convert the same boat to 114 L at 22.7 L/h and 40 km/h, giving 4.5 h and 180.8 km.
Burn input decides everything. Use cruise burn from the gauge or log, not the wide open figure from the brochure. Heavy load, foul bottoms, head seas, and long idles all raise burn above the calm water number.
Treat the reserve as a minimum. Inland lakes forgive a close call, while coastal and night runs do not. File a float plan and watch the gauge, since math cannot see weather.
How to use
- Pick imperial or metric first.
- Enter fuel aboard in the active units.
- Enter cruise burn per hour.
- Enter cruise speed.
- Read run time first, then range and usable fuel.
Formula
- fuel
- Tank fuel in gal or L.
- burn
- Cruise burn in gal/h or L/h.
- speed
- Cruise speed in mph or km/h.
The 10% reserve is a planning floor, not a legal rule. Offshore and river current trips often hold one third out, one third back, and one third spare. Burn should be gallons per hour at cruise, not top speed.
Worked example
Example: 30 gal day boat
30 gal fuel, burn 6 gph, cruise 25 mph (metric 114 L, 22.7 L/h, 40 km/h).
- Usable: 30 x 0.9 = 27 gal.
- Time: 27 / 6 = 4.5 h.
- Range: 4.5 x 25 = 112.5 mi.
- Metric: 102.6 L usable gives 4.5 h and 180.8 km.
- Tool shows 4.5 h, 112.5 mi, 27 gal.
Reference table
| Fuel | Burn | Range |
|---|---|---|
| 20 gal | 5 gph | 90 mi |
| 30 gal | 6 gph | 112.5 mi |
| 40 gal | 8 gph | 112.5 mi |
| 50 gal | 10 gph | 112.5 mi |
| 60 gal | 6 gph | 225 mi |
Limits
- Flat water math. Wind, waves, and current cut range.
- One burn rate only. Mixed speeds need splitting.
- 10% reserve is a floor. Offshore needs more.
- Ignores gauge error and unusable tank bottom.
Common mistakes
- Using tank size as usable fuel.
- Using top speed burn for cruise range.
- Forgetting idle and no wake time.
- Mixing mph with knots or km/h.
- Planning to the last gallon.
Try it yourself
Quick checks. Answers sit tucked under each question.
20 gal at 5 gph and 30 mph?
Usable 18 gal, 3.6 h, 108 mi.
50 gal at 6 gph, hours?
Usable 45 gal, 7.5 h at 6 gph.
Questions
How long can I run?
Usable fuel over burn. Here 27 gal at 6 gph is 4.5 h.
How far is that?
Time times speed. Here 4.5 h at 25 mph is 112.5 mi.
What is usable fuel?
Fuel minus a 10% reserve. Here 27 gal from 30.
Does it work in metric?
Yes. Enter liters, L per hour, and km/h for km range.
Does it cover weather?
No. Wind, waves, load, and idle time cut real range. Keep a wider margin offshore.
Is this a trip plan?
Refuel math, not navigation. Charts, tides, and no wake zones sit outside it.