Boat Trim Tab Size & Fit Calculator
Enter the boat length, drive layout, available transom width, and lower-unit clearance to turn the published trim-tab chart into a fit check for one tab on each side.
Recommended tab size24 x 9 in
- Fit status
- Tight fit
- Unused span per side
- 0
- Total tab area
- 432
- Span guideline coverage
- 100%
- Lower-unit clearance check
- Meets the 8 in minimum
- Best fitting alternative
- Not needed; preferred size fits
Matching chart rows
| Length range | Tab size | Per-side area | Fit |
|---|---|---|---|
| 19-24 ft | 18 x 9 in | 162 | Fits |
| 22-27 ft | 24 x 9 in | 216 | Tight fit |
Uses the published Bennett standard-tab chart rows that contain the entered boat length.
How to use this calculator
- Enter the boat length overall in feet.
- Choose the drive layout and whether the transom has standard space, limited space, or needs extra lift.
- Measure the clear mounting span available for one tab on one side of the transom.
- For outboards and I/Os, enter the planned clearance from the tab inboard edge to the nearest lower unit.
- Read the preferred chart size, fit status, combined tab area, and smaller fitting alternative when one exists.
How the trim-tab size is selected
This calculator encodes the Bennett Marine trim-tab sizing chart for conventional adjustable tabs. The chart gives span-by-chord sizes by boat length and drive layout, with separate 12-inch-chord choices for limited space or extra lift. It then checks whether the preferred chart size physically fits the clear span you measured on each side of the transom.
The lookup starts with the rows whose boat-length interval contains your entered length. Standard single outboard or I/O rows use 12 x 9, 18 x 9, 24 x 9, and 30 x 9 tabs across the 15- to 30-foot range. Twin outboard or I/O rows use 18 x 12 or 24 x 12 tabs across the 22- to 34-foot range. Inboard rows continue from 28 to 80 feet, generally with wider spans and 9- or 12-inch chords.
For a standard transom, the calculator prefers the widest 9-inch-chord row that applies to the entered length. For limited space or extra lift, it prefers an applicable 12-inch-chord row. When overlapping chart intervals both contain the length, the wider or larger-area chart choice is treated as the preferred recommendation.
The fit checks use these formulas:
M = A - S
T = 2 x S x C
G = S / L
- L is boat length in feet.
- A is clear transom span per side in inches.
- S is tab span in inches.
- C is tab chord in inches.
- M is unused span per side; negative means the preferred tab does not fit.
- T is the combined planform area of both tabs in square inches.
- G is coverage relative to the common one-inch-per-foot span guideline.
The fit status is "Does not fit" when the preferred tab span exceeds the measured clear span, "Tight fit" when the remaining width is less than one inch, and "Fits" otherwise. For outboard and I/O installations, the clearance check compares your tab-to-lower-unit distance with Bennett's 8-inch minimum.
What moves the result most
Boat length controls the chart row first. Drive layout matters because twin drives usually leave less uninterrupted transom width near the centerline. Clear span does not change the published preferred row, but it determines whether the preferred row can be installed and whether a smaller chart-listed alternative is available. Choosing limited space or extra lift favors a 12-inch chord, which can keep area up when span is constrained.
What this calculator leaves out
This is a manufacturer-guideline lookup, not a hydrodynamic lift model. It does not evaluate hull form, speed, center of gravity, actuator geometry, tab series, pocketed transoms, stepped hulls, surface drives, interceptors, or OEM-engineered systems. Before drilling, compare the chosen model with the manufacturer's installation template and confirm structure, fastener locations, actuator travel, wiring, and hydraulic or electric actuator requirements.
Worked example
Suppose a 24-foot single-outboard boat has a standard transom, 24 inches of clear span per side, and 8 inches from the planned tab edge to the lower unit. The overlapping Bennett rows that contain 24 feet are 18 x 9 and 24 x 9, so the wider 24 x 9 row is the preferred chart recommendation.
The fit margin is 24 - 24 = 0 inches, so the status is tight fit. Combined tab area is 2 x 24 x 9 = 432 square inches. Span coverage is 24 / 24 = 1.00, shown as 100% of the one-inch-per-foot guideline, and the 8-inch lower-unit clearance passes the published minimum.
Common questions
How do I measure the available transom span for trim tabs?
Measure one side at the intended tab mounting height, from the outside obstruction limit toward the drive or centerline obstruction. Exclude chines, strakes, swim platforms, drains, ladders, pockets, lifting strakes that cannot be bridged, and the required clearance from an outboard or I/O lower unit.
Why is a wider 9-inch-chord tab usually preferred?
For a conventional tab, more span spreads lift across more hull width and can work efficiently with less tab deflection. That is why the standard chart generally moves to wider 9-inch-chord tabs as boat length increases, when the transom has enough uninterrupted width.
When should I use a 12-inch-chord trim tab?
A 12-inch chord is commonly used when the transom cannot accept enough span or when the boat needs more lift from the same span. The tradeoff is that chord does not solve every clearance or actuator problem, so the full installation template still matters.
How far should trim tabs be from an outboard lower unit?
Bennett publishes an 8-inch minimum distance from the inboard edge of the tab to the lower unit for outboard and I/O installations. This calculator reports that clearance separately because a tab can have enough span on paper and still be too close to the drive.
Do twin-outboard boats need smaller trim tabs?
Often, yes. Twin outboards or twin I/Os take up more usable transom width near the centerline, so the published twin-drive rows use shorter spans with 12-inch chords for the length ranges covered.
Are trim tabs and interceptors sized the same way?
No. Interceptors create lift differently and have their own manufacturer sizing rules. Use this calculator for conventional adjustable trim tabs only, not interceptors or automatic self-leveling products unless the maker says the same chart applies.