One inch of fin per foot of board. Nine-foot board, nine-inch fin. Everybody says it, we have said it, and Cleanline Surf says it for everyone: "a good rule is to go one inch of fin for every foot of board length."

Nobody checks it against the people who make fins.

True Ames publishes three board-length sizing charts: one for all-around singles, one for flex fins, one for noseriders. Twelve rows between them. The inch-per-foot answer lands below the midpoint of the published range in all twelve. Not once above. On a 6'6" board the rule says 6.5" and True Ames' range starts at 7". On a 7'6" board the rule says 7.5" and the range starts at 8". On a 10'0" noserider the rule says 10" and the chart's ten-foot-and-over row starts at 10.5".

The rule doesn't just fail. It fails in one direction — it hands you a fin that is too small — and it gets worse the shorter your board is. The gap to the middle of True Ames' band is 0.25" on a 9'6" log and 1.25" on a 6'6" mid-length. Backwards, because mid-length riders are the ones asking.

Cleanline hedges it in the next breath — "of course, this is only a guide; there are no rules, remember." Fair. But a guide that misses in the same direction twelve times out of twelve is not a tolerance. It's a bias, and you can correct for a bias.

This post is about the number, not the template. For template, our longboard single fin guide sorts the 9"-and-deeper templates into pivot, all-round and flex, and the mid-length post takes the shorter boards. This one supplies the arithmetic they skip, and ends in a size.

Why depth is the only number you get

Pick up a side bite and the page gives you three numbers — height, base, area. Pick up a single fin and you get a depth. That is it.

The reason is hardware. A single fin goes in a box, and True Ames sells the box and prints its limits: the 10.5" box "accepts all fins up to 9.75" base length," the 8.5" box up to 8".

So the base is capped by a routed slot and depth is not. Almost everything a single fin can do to get bigger, it has to do by getting deeper — which is why the range runs as a ladder of depths, and why the only number on the label is the one the box lets the designer move freely.

Which matters, because the two do different jobs. Depth buys hold; base buys drive. Depth reaches into water the board has not disturbed yet, so it is the part still working when the tail goes light. Base lies along the board, low and forward, in water the board is already driving through — that is what you feel coming off the bottom.

The label gives you the first and says nothing about the second.

Same depth, a fifth more fin

Six of True Ames' single-fin designs state a base in prose, and they settle it. The Troy Elmore Flex is 10" deep on "a wide 8" base." The D-2 is 9" deep on a base wider than the fin is tall, while the Fantastic Acid Wide Base is half an inch deeper on a base nearly half again narrower.

Size by depth and you would call the D-2 the smaller fin.

Gato Heroi shows what that costs, because ten of the twelve we carry publish a base and six templates are sold at the same 9.5":

Gato Heroi template at 9.5" Published base
C Fin 5.9–6.1"
T Fin 6.2–6.5"
Killer Fin 6.3"
B Fin 6.6–6.8"
D Fin 7.2–7.5"
A Fin not published

Same depth, same maker, one shop, and the widest published base is 27% wider than the narrowest. "9.5 inches" can mean a fifth more fin or a fifth less than the one next to it on the rack — not by mistake, since the C Fin is drawn for high-performance mid-lengths and the D Fin is a zero-rake pivot for vintage logs. The number on the box just does not say so.

So use the depth to get in the right band, and the maker's description to pick inside it.

Diagnose what you have before you buy anything

Too much fin: the board tracks. It holds a line you did not ask for, feels stiff off the bottom, will not come around in the pocket — you start the turn and the tail follows late and wide. Most people read that as needing a better fin. It usually needs a smaller one, or the same one further forward.

Too little fin: the tail slides on a steeper drop, there is no drive off the bottom, and the board feels vague in textured water rather than loose. Loose is a turn you can start. Vague is one that does not take.

Either way it is not a template problem. Change the size, or change the position.

And the position is free. Where the fin sits in the box changes what it does — forward is looser and easier to redirect, back is more hold and more drive. Half an inch of travel changes the feel more than half an inch of fin does.

Get inside the published range first, then use the box. Two cases where you do not get the travel: the Greenough Stage III "runs the entire length of the fin box, and does not accommodate adjustments or leash knots," and an 8.5" box has less travel than a 10.5" one before you put anything in it.

Two things that move you inside the range

Tail width. A wide, flat tail is a lot of planing surface a long way from the stringer. Push on it and it generates lateral force fast — the tail wants to slide before the rail has finished biting, and the fin is the only thing arguing. A pulled tail has less out there and asks the fin to do less. Wide or square tail, more fin. Pulled tail, less. That is mechanism, not a spec; there is no tail-width figure to look up.

Rider weight. More mass is more force through one fin, and more force needs more area to resist it.

No fin maker publishes an adjustment for it. True Ames names rider weight in the footnote on all three charts and then gives no figure anywhere in its 466 products. The only published number we could find is a retailer's, and it is half an inch to a full inch either way — which is what the split in the tables below uses, as a rule of thumb rather than a measurement.

One more thing the label hides: going up the ladder is usually scaling a template, not adding depth. Half an inch on a wide-base template is a bigger move than half an inch on a narrow one, and nothing on the box says which yours is. True Ames states it outright on exactly one single fin, the Tyler Warren Flux: "The two sizes have slightly different templates."

The sizing tables

Read the assumptions, because they are what you would be arguing with.

The depth ranges are True Ames' published bands, copied from their All Around, Flex and Noserider pages. The split inside each band is ours — pulled tail or lighter rider low, wide or square tail or heavier rider high, at about half an inch. Disagree with the split and you are disagreeing with us, not with True Ames.

Two products per cell because the depth is the decision. Either is the right size for that board, and choosing between the templates is an argument about outline. Our longboard single fin guide sorts the 9"-and-deeper templates into pivot, all-round and flex, and says what each group costs you.

Prices are ours, taken 2 August 2026. Four sizes show a range, because one colorway is priced apart from the rest.

All-around and flex singles

Board True Ames' band Pulled / round tail, lighter rider Wide / square tail, heavier rider
6'–7' 7"–8.5" 7"–7.5" — California Classic 7" $86.75 · Greenough 4-A 7" $85.75–87.00 8"–8.5" — Yater Classic 8" $92.75 · L-Flex 8.5" $89.75–95.75
7'–8' 8"–9.25" 8"–8.5" — Lovelace Thicklizzy 8" $92.75 · Greenough 4-A 8" $92.00 8.5"–9" — California Classic 8.5" $95.75 · Channel Islands Spicy 9" $99.50
8'–9' 8.5"–9.75" all-round, 8.5"–9.5" flex 8.5"–9" — Yater Classic 8.5" $95.75 · California Classic 9" $99.50 9"–9.5" — Greenough 4-A 9" $98.00–100.00 · California Classic 9.5" $103.50
9'–10' 9"–10.5" 9"–9.5" — The Slick 9" $99.50 · Velzy Classic 9.5" $105.00 10"–10.5" — Greenough 4-A 10" $103.00 · Wayne Rich Harmonic '67 10.5" $111.50

Noserider templates

True Ames publishes a separate, deeper chart for these, and the gap is the argument for reading it separately: a 9'8" board gets 9"–10.5" as an all-rounder and 9.75"–10.75" as a noserider — the same board, three-quarters of an inch more fin at the floor. The chart's rows overlap at 9'6", which sits in two at once; take the deeper one if you are noseriding.

Board True Ames' band Pulled / round tail, lighter rider Wide / square tail, heavier rider
9'–9'3" 9"–9.75" 9"–9.5" — Velzy Noserider 9" $99.50 · The Slick 9.5" $103.50 9.5"–9.75" — Nat Young 9.5" $103.50 · Heritage 9.75" $103.50
9'4"–9'6" 9.5"–10" 9.5" — Yater Apocalypse 9.5" $103.50 · Velzy Noserider 9.5" $103.50 9.75"–10" — Heritage 10" $107.50 · Justin Quintal Pivot 9.75" $103.50
9'6"–10' 9.75"–10.75" 9.75"–10" — Tyler Warren Pivot 9.75" $103.50 · Barrett Miller Ace 10" $107.50 10.25"–10.5" — Tyler Warren Pivot 10.25" $107.50 · Heritage 10.5" $111.50
10'+ 10.5"–10.75" 10.5" — Hobie Legacy 10.5" $113.00 · Greenough Stage III 10.5" $111.50–116.50 10.5"–10.75" — Greenough 4-A 10.5" $110.00 · Tyler Warren Pivot 10.75" $111.50

Two rungs are thin. No all-round template is made at 7.75" or 9.25". At 7.75" the pick is the Derrick Disney Farm Single at $92.75, drawn "for eggs, gliders, and pintail single-fin boards" on a published 5 7/8" base. At 9.25" take the Skip Frye or the Justin Quintal Cutty, both $103.50. Otherwise round to the nearest half inch.

And the 4-A skips two. The Greenough 4-A is sold in nine depths — 6.5, 7, 7.5, 8, 8.5, 9, 9.75, 10, 10.5 — with no 9.25" and no 9.5", on True Ames' listing or ours. Every step is half an inch except the two either side of 9.75" — three quarters up to it, a quarter past it. If the answer for your board is 9.5", the 4-A isn't made in it; the California Classic and the Yater Classic both are, at $103.50.

Three boards, three numbers

A 9'6" log you want to noseride. The rule says 9.5". 9'6" sits in two rows of True Ames' noserider chart; noseriding takes the deeper, 9.75"–10.75". Take the Heritage at 10", $107.50, which True Ames calls "full of area and minimal rake… built for noseriders." It isn't made below 9.75".

Pulled-tailed log, lighter rider: go to the floor of that band and not under it. Take the Tyler Warren Pivot at 9.75", $103.50, a quarter inch shallower. True Ames puts its hold in "a broad base and upright profile": base, named out loud on a fin in the table above. The Heritage is the wrong pick if you want to turn off the tail in the pocket — a lot of area with no rake in it, so it holds where you were asking it to let go.

A 7'6" mid-length. The rule says 7.5". True Ames says 8"–9.25". Take the Greenough 4-A at 8", $92.00, or — wide tail or heavier rider — the California Classic at 8.5", $95.75. That's half an inch to a full inch more than the rule hands you, and it's the most common under-finning we see.

A 2+1 you took the center too far down in. You read "drop an inch or two," went from a 9" single to a 6" center, and now there's no drive through the flat. True Ames' 2+1 chart gives a mid-length and a performance longboard the same center band, 6.5"–8"; 6" is under the floor of both. Go to 7" — the Cutaway at $86.75, where True Ames names 7", 7.5" and 8" as its own most popular 2+1 sizes, or the Wayne Rich Power Tilt at the same depth and price. Our side bites guide covers the rest.

One last thing

Every number above has a range on it, because fin sizing is not precise enough to deserve a single figure.

A fin half an inch off the ideal is not what's holding your surfing back. Changing it every third session is.

Sources

All True Ames figures retrieved 2 August 2026: the three board-length charts and the terminology at https://www.trueames.com/pages/all-around-fins, https://www.trueames.com/pages/noserider-fins, https://www.trueames.com/pages/flex-fins, https://www.trueames.com/pages/fin-terminology and the 2+1 chart at https://www.trueames.com/pages/2-1-setup-fins; box limits from the fin box listings; bases, ladders and designer copy from product pages via products.json. Gato Heroi bases from our own product pages. The rule and the weight adjustment from https://www.cleanlinesurf.com/blogs/surf/how-to-pick-a-single-fin-for-your-surfboard and https://boardexchange.pt/blogs/news/how-to-choose-a-longboard-fin. Prices from our catalog, same day. Percentages and midpoints are arithmetic on those figures. Full working on file.