What is the rule of thirds for fuel?

The rule of thirds means using one-third of your fuel going out, one-third for the return, and keeping one-third in reserve. It gives a 33% safety buffer — the most conservative common guideline for offshore passages.

Should I use knots or mph?

Knots are the standard for marine navigation because one knot equals one nautical mile per hour, so the math stays consistent. Use mph if your instruments only show statute miles per hour — the calculator handles both and shows range in both units.

Does current or wind change the result?

This calculator uses your input speed and burn rate and assumes calm conditions. A headwind or opposing current reduces effective speed without reducing fuel use — in those conditions, increase your reserve or reduce your planned range.

Why use a percentage reserve rather than a fixed gallon amount?

A percentage reserve scales with tank size: 25% of a 200-gallon tank provides more absolute buffer than 25% of a 50-gallon tank, but both give the same proportional safety margin relative to burn rate and range.

How do I find my burn rate?

Check your engine manufacturer's fuel consumption chart at your target RPM, use an onboard fuel-flow meter reading, or track fuel consumption over a logged run at known speed. When in doubt, use a slightly higher number to keep estimates conservative.

Marine Fuel Range Calculator

Name: Marine Fuel Range Calculator
Availability: InStock
Author: Nham Vu

Enter fuel capacity, burn rate, and cruising speed to get your vessel's safe range in nautical miles and statute miles, with one-click reserve presets.

Vessel Parameters

Fuel Unit

Speed Unit

Tank Capacity (gal)

Safety Reserve

Custom: %

Fuel Burn Rate (GPH)

Cruising Speed (knots)

Enter your vessel parameters and click Calculate Range.

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Summary

Enter fuel capacity, burn rate, and cruising speed to get your vessel's safe range in nautical miles and statute miles, with one-click reserve presets.

How it works

Select your fuel unit (US gallons or liters) and speed unit (knots or mph).
Enter your total fuel tank capacity.
Choose a safety reserve preset — rule of thirds (33%), standard 25%, or a custom percentage.
Enter your engine's fuel burn rate at cruising throttle (GPH or LPH).
Enter your cruising speed. The calculator shows safe range in nautical miles and statute miles, hours underway, usable fuel, and reserve held.

Use cases

Verify your vessel can complete an offshore passage before casting off.
Plan a round trip and confirm enough fuel remains for the return leg.
Compare range at different throttle settings to find the most fuel-efficient cruising speed.
Quickly convert a fuel range estimate between nautical miles and statute miles.
Set a conservative reserve for unfamiliar waters or areas with few fuel stops.
Brief crew on range limits using a printed or copied summary.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu