Undersized trim tabs are the single most common mistake we see when a customer calls us frustrated that their new tabs “don't do anything.” Nine times out of ten the tabs work fine. They're just too small for the boat. A 24-foot center console running 12x9 tabs will plane flatter and burn less fuel than the same boat fighting to get up on a pair of 9x9s, and the difference shows up the first time you load four guys and a full cooler over the transom.
There's no magic to picking the right size. The industry has settled on a simple starting formula, and once you understand the two numbers that matter — span and chord — you can size a set in about thirty seconds. This guide is the calculator. We'll walk through the math, the exceptions that trip people up, and the real-world adjustments we make in the shop when the textbook answer doesn't fit the transom.
The Two Numbers That Matter: Span and Chord
Every trim tab is described by two measurements, and the order matters depending on which brand's catalog you're reading, so always confirm which is which.
Span is the side-to-side width of the tab — how far it runs across the transom. This is the number that does the heavy lifting. Literally. Span has the biggest effect on how much lift a tab generates, because lift is a function of surface area working against the water, and widening the span adds area without pushing the tab deeper into the flow where it creates drag.
Chord is the fore-to-aft depth — how far the tab sticks out behind the transom. The two standard chords are 9 inches and 12 inches. A 9-inch chord is what most planing boats want. The 12-inch chord exists for specific cases we'll cover below, not as a default upgrade.
So a “24x9” Bennett is 24 inches of span, 9 inches of chord. A Lenco “9x12” is listed chord-first: 9-inch chord, 12-inch span. Same physical concept, reversed naming. We've watched people order the wrong tabs because they assumed both brands list dimensions the same way. They don't.
The Core Formula: One Inch of Span Per Foot of Boat
Here's the rule both Bennett and Lenco publish, and it's the right place to start every calculation:
Use at least one inch of trim tab span (per side) for every foot of boat length.
A 22-foot boat needs a minimum of 22 inches of span per side, which rounds up to a 24-inch tab. A 30-footer wants 30 inches of span — call it a 30x9 or, if the transom won't take that width, a wider-chord compromise. A 17-foot bay boat lands around 18 inches of span.
Two things people miss about this rule. First, it's a minimum, not a target. The largest tab that fits cleanly on your transom is almost always the most efficient choice, because bigger tabs do the same work with less downward deflection, which means less drag at cruise. Second, the per-foot rule assumes a typical planing hull at typical power. Heavy boats, slow boats, and oddball transom layouts all push you toward more area.
Quick-Reference Sizing Table
| Boat Length | Minimum Span (per side) | Typical Lenco Kit | Typical Bennett Kit |
|---|---|---|---|
| 14–17 ft | 9–12 in | 9x9 Standard | 9x9 / M120 |
| 18–21 ft | 12 in | 9x12 Standard | 18x9 / 18x12 |
| 22–25 ft | 12–24 in | 12x12 Standard | 24x9 |
| 26–32 ft | 24–30 in | 12x18 Standard | 30x9 |
| 33–40 ft | 30–36 in | Heavy-duty / Edge Mount | 30x12 |
| 41 ft+ | 36 in+ | Multiple actuators per tab | 36x12 / custom |
Treat this as the starting point, then adjust for the factors below. The boat's behavior on the water is the final authority, not the chart.
When You Need a 12-Inch Chord Instead of 9
Most of the time, you gain lift by adding span and keeping the 9-inch chord. The 12-inch chord is the tool you reach for when you can't add span — or when the boat simply needs more lift than span alone can deliver.
We go to a 12-inch chord in four situations:
A transom that physically limits how wide you can mount. Twin outboards, a swim platform, a jack plate, or a transom-mounted bracket can eat up the real estate you'd otherwise fill with span. When you can't go wider, you go deeper. A 24x12 generates noticeably more lift than a 24x9 because you've added a third more surface area.
Slower boats running under 15 knots. Displacement and semi-displacement hulls, heavy trawlers, and pontoons don't have the water speed to make a shallow tab work hard. The bigger chord gives them the area they need at low velocity.
Boats over 50 feet, where the sheer mass demands every square inch you can bolt on. And outboard brackets or setback configurations that move weight aft and need aggressive lift at the very back of the boat.
The trade-off is real: a 12-inch chord that's oversized for a fast hull can over-correct and make the boat feel twitchy, and it adds drag if you're constantly running the tabs near neutral. For a 21-foot bass boat that runs 55 mph, a 9-inch chord is the right call. Don't upsize the chord because more sounds better.
Real Adjustments We Make in the Shop
The formula gets you 90% of the way. These are the corrections we apply for the last 10%, and they come from actually bolting tabs onto boats, not from a spec sheet.
Heavily loaded boats go up a size. If you run a 25-foot center console offshore with a full fuel load, a tower, livewells, and five anglers, size as if the boat were 28 feet. We've seen too many “correctly sized” tabs come up short because the owner didn't account for how he actually loads the boat.
Repowering to four-strokes adds stern weight. Modern four-stroke outboards are heavy, and a repower from a two-stroke often adds 150–250 lbs hanging off the transom. That weight sits exactly where trim tabs do their work, so a boat that planed fine on small tabs may want a larger set after a repower.
Outboards want the tabs mounted close to the chines. Span only helps if the water actually reaches the tab. On a deep-V with a lot of deadrise, push the tabs outboard toward the chines where there's clean flow, and don't let the outboard's wash blank them out.
When in doubt, size up — within reason. A slightly oversized tab simply runs with less deflection. A slightly undersized tab runs maxed out and still won't flatten the boat. The penalty for going one size too big is minor; the penalty for going too small is a boat that never trims right. That's not a close call.
Lenco vs Bennett: Does the Brand Change the Math?
The sizing math is identical — both publish the same one-inch-per-foot rule. The difference is mechanism, and it affects which kit you buy at a given size, not the size itself.
Lenco tabs are electric, driven by a stainless actuator with a reputation for fast response and dead-simple wiring. There's no hydraulic pump to leak and no fluid to bleed. For most recreational boats up to the mid-30s, a Lenco electric kit is what we install most often. If you want the full rundown on their actuator and finish options, our Lenco Electric Trim Tab Review breaks down the Sport Tab versus Standard differences.
Bennett built its name on hydraulic systems, and for bigger boats and commercial work, hydraulics still have an edge in raw holding power and longevity. Their classic kits run from 12x9 up to 30x12.
Either way, get the size right first. A perfectly chosen brand in the wrong size still gives you a boat that won't plane flat.
Here's where current pricing lands for the sizes most of our customers buy. These are complete kits — tabs, actuators, and a switch — and reflect mid-2026 pricing.
| Kit | Span x Chord | Fits | Est. 2026 Price |
|---|---|---|---|
| Lenco Standard Electric Kit 9x9 | 9 x 9 in | 14–18 ft | $719 |
| Lenco Standard Electric Kit 9x12 | 12 x 9 in | 18–24 ft | $736 |
| Lenco Standard Electric Kit 12x12 | 12 x 12 in | 22–30 ft | $752 |
| Bennett Classic Hydraulic Kit 24x9 | 24 x 9 in | 22–28 ft | $875 |
| Bennett Classic Hydraulic Kit 30x12 | 30 x 12 in | 32–40 ft | $927 |
Prices move with stainless and shipping costs, so check the live product page for the current number. The kit you want is the one that matches your measured transom, not the cheapest box on the shelf.
How to Use the Calculator: A 30-Second Walkthrough
- Take your boat length in feet. That's your minimum span per side, in inches.
- Round up to the nearest available tab size.
- Measure your usable transom width per side, between the keel/strakes and the chine or the outboard. If it won't accept your calculated span, drop to the next span down and step the chord up to 12 inches to recover the lost lift.
- Add a size if the boat runs heavy, carries a tower, or just got repowered to four-strokes.
- Default to a 9-inch chord unless you hit one of the four 12-inch-chord cases above.
That's it. The boat will confirm you got it right the first time you trim down at cruise and watch the bow settle without the engine straining.
Bottom Line
Start with one inch of span per foot of boat, round up, and only deviate from the 9-inch chord when the transom forces your hand or the boat needs lift it can't get from span. The most common failure isn't picking the wrong brand — it's buying tabs that are too small because they were cheaper or because the owner sized for an empty boat. Measure your transom, account for how you actually load the boat, and lean toward the larger set when you're between sizes. A boat with correctly sized tabs planes faster, runs flatter, and burns less fuel, and you'll feel the difference on the first ride.
👉 Shop All Trim Tabs at NVN Marine →
Frequently Asked Questions
What size trim tabs do I need for a 24-foot boat?
Start with 24 inches of span per side, which means a 24x9 Bennett or a Lenco 12x12 (12-inch span, 12-inch chord) for most 24-footers. If the boat runs heavy — tower, full fuel, offshore loadout — size up toward what a 26–28 footer would use. Measure your usable transom width before ordering to confirm the span will fit between the chine and the outboard.
Is span or chord more important for lift?
Span. The side-to-side width of the tab has the biggest effect on how much lift it generates, because you're adding surface area without pushing the tab deeper into the water and creating drag. The standard approach is to maximize span first at a 9-inch chord, and only go to a 12-inch chord when you can't fit the span you need.
When should I use a 12-inch chord instead of a 9-inch?
Use a 12-inch chord when the transom limits how wide you can mount (twin outboards, brackets, swim platforms), on slower boats running under 15 knots, on boats over 50 feet, and on setback or bracketed outboard installs that need aggressive lift at the stern. For a fast planing hull with room for span, stick with the 9-inch chord to avoid over-correction and extra drag.
Can trim tabs be too big for my boat?
Yes, but the penalty is mild compared to going too small. Oversized tabs simply run with less deflection and can feel slightly twitchy on a fast, light hull if you over-deploy them. Undersized tabs run maxed out and still won't flatten the boat. When you're between sizes, going up is the safer mistake.
Do Lenco and Bennett use the same sizing rule?
Yes — both publish the one-inch-of-span-per-foot-of-boat-length rule, so the size you calculate is the same regardless of brand. What differs is the mechanism: Lenco runs electric actuators, Bennett is known for hydraulics. Just watch the catalog order, because Lenco often lists chord first (9x12 = 9-inch chord, 12-inch span) while Bennett lists span first (24x9 = 24-inch span, 9-inch chord).
Will bigger trim tabs improve fuel economy?
Correctly sized tabs almost always improve economy because they let the boat plane at a lower speed and run flatter at cruise, reducing the bow-high drag that wastes fuel. The gain comes from proper sizing, not just bigger tabs — an oversized set you constantly run near neutral can add a little drag. Size to the boat and its real-world load, and the efficiency follows.
