Boat Turnaround Range Calculator
Enter your measured fuel, burn rate, cruise speed, and the expected current for each direction to find the farther safe turnaround point. The result protects unusable fuel, non-cruise use, and the reserve before splitting trip fuel equally between the outbound and return legs.
Maximum one-way turnaround distance (nm)54.03
- Maximum round-trip distance (nm)
- 108.05
- Limiting leg
- Outbound
- Outbound and return ground speeds
- 18 kn outbound; 22 kn return
- Planned trip travel time
- 4 h 2 min
- Planned trip fuel (gal)
- 45.4
- Estimated trip fuel cost
- $204.32
- Fuel remaining at return (gal)
- 54.6
- Reserve margin (gal)
- 19.63
- Planned trip status
- Planned distance fits both equal fuel allocations
Each travel leg receives 30.02 gal after unusable fuel, the non-cruise allowance, and the reserve are protected.
Current is treated as constant and directly aligned with the route on each leg.
How to use this calculator
- Choose gallons or litres, then enter the fuel you have actually measured aboard.
- Set aside unusable fuel, non-cruise fuel, and the reserve you plan to keep untouched.
- Enter the cruise speed and fuel-burn rate measured at the same operating point.
- Enter signed current for each leg and the proposed one-way distance.
- Read the limiting leg and trip status before treating the calculated distance as a planning limit.
How boat turnaround range is calculated
Range planning starts with the fuel that can actually be used, not the tank’s rated capacity. The calculator subtracts the unusable fraction and the allowance for warm-up, idling, maneuvering, a generator, or fishing. It then protects the chosen reserve before dividing the remaining trip fuel equally between the outbound and return legs. That equal split makes the result a turnaround limit rather than an optimistic total-range estimate.
The calculation uses these relationships:
E = F × (1 − u) − A
q = E × (1 − r) ÷ 2
dmax = min(q × Vo ÷ B, q × Vr ÷ B)
- F is fuel on board, u is the unusable-fuel fraction, and A is the non-cruise allowance.
- E is fuel remaining after those deductions, r is the reserve fraction, and q is each leg’s equal fuel allocation.
- Vo and Vr are outbound and return ground speeds: speed through the water plus signed current.
- B is cruise fuel burn per hour, and dmax is the maximum one-way distance in nautical miles.
For a proposed distance, the tool separately calculates each leg’s time and fuel. The plan passes only when both legs stay within their own equal allocations. This matters with current: a fast following-current leg can leave the total fuel use and overall reserve margin looking acceptable even while the slower leg exceeds its share.
What changes the result most
Fuel burn, speed through the water, and the slower leg’s current have the strongest direct effects. A current is added to the speed through the water, so an opposing current reduces ground speed and makes that leg consume more fuel per nautical mile. Fuel reserved for activities outside cruise also reduces both leg allocations. A 33.3% reserve corresponds to the familiar rule of thirds: one third out, one third back, and one third held in reserve.
Use a vessel-specific burn rate at the cruise setting you intend to run. ICOMIA’s fuel-consumption standard notes that boat, water and weather conditions, loading, engine and propeller settings affect performance. Logged fuel added against hours run, or a properly used flow meter, is more useful than a generic engine figure.
What this calculator leaves out
The model treats current as constant and directly aligned with the route. It does not model cross-current, tide changes, wind, waves, sea state, route detours, alternate destinations, loading, hull fouling, trim, engine condition, or changes in speed and burn during the passage. It is a fuel-planning check, not a passage plan or a guarantee that conditions will remain safe.
Worked example
Suppose you have 100 gallons aboard, set aside 5% as unusable and 5 gallons for non-cruise use, and keep a 33.3% reserve. At 20 knots through the water, a 2-knot opposing outbound current gives 18 knots over ground; a 2-knot following return current gives 22 knots. With a 10 gallon-per-hour burn rate, usable fuel after the allowance is 100 × 0.95 − 5 = 90 gallons and each leg receives 90 × 0.667 ÷ 2 = 30.015 gallons.
The outbound leg limits the trip: 30.015 × 18 ÷ 10 = 54.027 nautical miles, compared with 66.033 nautical miles on return. A proposed 40-nautical-mile turnaround takes about 4 hours 2 minutes, uses 45.40 gallons including the allowance, and leaves 54.60 gallons at return. Its reserve margin is 19.63 gallons, and both individual leg allocations pass.
Common questions
What is the rule of thirds for boat fuel?
The traditional rule of thirds assigns one third of fuel to the outbound trip, one third to the return, and leaves one third in reserve. It is a simple planning rule, not a substitute for adjusting the reserve when weather, current, distance to shelter, or uncertainty call for more margin.
Should boat range use speed through the water or speed over ground?
Fuel burn is normally tied to engine setting and speed through the water, while travel time depends on speed over ground. This calculator starts with speed through the water and adds the signed along-route current to calculate the ground speed separately for each leg.
Why can one leg limit the turnaround distance?
An opposing current lowers the ground speed for that direction, so the boat runs longer and burns more fuel per nautical mile. The calculator gives outbound and return equal fuel allocations, so a slow leg can fail even if the total trip fuel still appears to leave a reserve.
How do I measure my boat’s real fuel-burn rate?
Use a flow meter or log fuel added and engine hours across several trips at the same cruise setting. Recheck the number after a major change in loading, propeller, hull condition, or operating speed, because a published engine figure may not match the installed boat.
Does this account for wind, waves, load, and hull condition?
No. Those conditions can change both speed and burn rate substantially, and current can also vary with place and time. Enter a more conservative burn rate or reserve when conditions are uncertain, and use current weather and tide information in a proper passage plan.