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.

It isn't the only answer. The 4A has rake and a tapering, moderately flexible tip, and True Ames designed it for all board shapes and surf conditions. Three other places to look: the Power Blade, built to flex and twist at the head so the tail follows the nose like a thruster; the Power Glide, which has more rake, more area and more flex than the Power Blade for a wider turning radius; and the Lovelace Thicklizzy, a wide-based single with a narrow, flexy tip, suited to mini gliders and single fins of all kinds.

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. Its heavier glass lets the fin flex 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)
  • Side bites: 2.6" to 4.0". Start with the 3.7"; drop to the 3.25" to keep more of the single fin's feel, or go up to the 4.0" in steeper waves. The side bites guide covers every size and template.

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

Start with the fin centered in the box and move it from there. If you want a measured starting point, one rule of thumb: lay the fin flat on the bottom of the board, base on the stringer line, and slide it until about a third of its height hangs past the rail. It was written for a board with a 10" single-fin box, so on a shorter box treat it as a place to begin, not an answer.

Decision Framework

  1. Identify the board: egg or speed shape
  2. Choose single or 2+1 (2+1 is often the OB answer)
  3. Pick one baseline fin: 4A in the size band above
  4. Start centered in the box. Feels stiff? Move forward ½". Feels slidey? Move back ½"
  5. 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