Mid-lengths are everywhere right now, and fin advice is a mess. One person says one inch per foot. Another says size down. Someone else says sidebites are training wheels. Then you paddle out at Ocean Beach in cold, choppy water and realize your board doesn't care what the internet thinks.
Here's why it's confusing: "mid-length" is a size range, not a design. A 6'8" egg and an 8'0" speed shape share a category name, not a fin job.
Pick the Board Type First
Fin rules only work when boards are comparable. Mid-lengths aren't. Fuller outlines want trim and glide. Pulled-in outlines want rail hold and response. Same fin, different result.
Eggs and Fuller Outlines
More foam, fuller template, surfed flatter. You're trimming, drawing lines, making speed with positioning—not pumping.
Speed Shapes and Pulled-In Mid-Lengths
Narrower template, more tail curve, surfed off rail. You're pushing harder and redirecting faster, especially in beachbreak sections.
2+1 vs Pure Single
Pure single = clean flow. 2+1 = more hold and a wider usable range, which is often the right call for Ocean Beach bump and late drops.
OB reality: it's powerful, cold, and frequently textured. If you're mostly surfing OB, favor hold and predictability over ultra-loose.
Fin Recommendations by Board
If you want one go-to fin for mid-lengths, it's the Greenough 4A. True Ames specifically notes that 7.5"–9" pairs well with speed shapes and mid-lengths.
Eggs
Eggs want a fin that trims fast and turns predictably. The main mistake is going too small and ending up with a vague, trackless feel in textured water.
- 6'8"–7'2" egg: start at 7.5" 4A
- 7'4"–7'10" egg: start at 8" 4A
- 8'0" egg: start at 8.5" 4A
If you want more spring through turns, swap to the 4A Volan—the thicker layup flexes through the body, not just the tip.
Speed Shapes
Speed shapes tolerate a more performance-oriented single. Do you want classic flow or thruster-like response?
- Flow and drawn-out lines: 4A, usually 7.5"–8.5"
- More response off the tail: Power Blade
OB note: when it's fast and a little overhead, get hold from position first—move the fin back—before sizing up. A half-inch back often solves what people think requires a bigger fin.
2+1 Setups
The most common 2+1 mistake is over-finning the center. The board gets stiff, you lose speed through turns, and you start shopping for a new board instead of fixing the setup.
- Center fin: 6.5"–7.5" (start at 7" for most 7'0"–8'0" mid-lengths)
- Sidebites: keep them small, roughly 3.25"–3.75"
Why One-Inch-Per-Foot Is Just a Starting Point
It's a memory trick, not a law. Height is only part of it—base, rake, and flex matter just as much.
- Wide tail or lots of planing area: more base or a touch more size
- Pulled-in tail, surfed off rail: you can often run less fin than the rule suggests
- Running sidebites: size the center down
Simplest answer: most mid-length singles land in the 7.5"–8.5" range on a 4A. Start there and adjust with the box.
Box Position Basics
Half-inch changes matter. Make small moves, then surf it.
- Forward: looser, easier to redirect, less hold
- Back: more hold, more drive, more drawn-out turns
A baseline many shapers use: set the fin so roughly one-third of the fin height extends past the rail when laid out to either side. Fast way to get to a sensible starting position.
Decision Framework
- Identify the board: egg or speed shape
- Choose single or 2+1 (2+1 is often the OB answer)
- Pick one baseline fin: 4A in the size band above
- Start centered in the box. Feels stiff? Move forward ½". Feels slidey? Move back ½"
- Change one variable per session. Fin or position or sidebites. Never all three.
The Two-Session Test
Session 1: run your baseline setup, don't touch it mid-session. Notice two things only—do you feel slip on steeper drops, or do you feel stuck through cutbacks?
Session 2: make one adjustment. Slip = move back. Stuck = move forward. If you're still unhappy after that, then change fin size or template. Not before.
Bring your board dims to 4051 Judah. Tell us where you're surfing and how you want it to feel. We'll get you on a fin that makes sense.
Sources: Swaylocks mid-length fin threads, True Ames 4A specs