Fly Tying Bead Head Size Guide

Why Bead Head Sizing Matters

The bead head must fit snugly against the hook eye without being so large it slides over the bend or so small the hook eye disappears inside the bead. The bead hole must be smaller than the hook wire at the bend (to hold the bead in place) but large enough to thread the hook point through during assembly.

Most commercial tungsten bead sizing charts match bead outer diameter to hook gap width. As a rough guide, bead outer diameter should be roughly equal to or slightly larger than the hook gap width (the distance between hook shank and point tip).

Tungsten vs Brass

Tungsten density: approximately 18.0 g/cm3. Brass: approximately 8.5. Glass: approximately 2.5. For the same bead size, tungsten weighs about 2.1 times more than brass.

In water, what matters is buoyant weight, which is density minus the 1.0 g/cm3 the bead displaces. That works out at (18 − 1) ÷ (8.5 − 1) = 2.3 to 1 in tungsten’s favour.

Sink rate is not the same ratio, and this is where most bead charts overclaim. A sinking bead reaches terminal velocity when drag balances buoyant weight, and drag rises with the square of speed. So 2.3 times the pull buys the square root of that in speed, roughly 1.5 times faster, not the “twice as fast” the number invites you to say. Over a two-metre drop in a run, 1.5x is still the difference between fishing the seam and drifting over it.

How the weights below are worked out

A bead is a sphere with a hole through it, so its volume is the napkin-ring solid: (π/6) × h³, where h = 2√(R² − r²), R is the bead radius and r is the bore radius. Bores used here run from 0.8 mm on the smallest bead to 2.0 mm on the largest. Treating a bead as a solid sphere overstates a 3.3 mm tungsten head by about a quarter, which is enough to matter when you are deciding whether a nymph will hold the bottom.

Brand tolerances move these by 10% either way. Use them to compare materials, not to weigh a box.

Standard Sizing Chart

Hook 22-18: 2.0-2.4 mm bead. Hook 16-14: 2.8 mm. Hook 12-10: 3.2-3.5 mm. Hook 8-6: 4.0-4.5 mm. Hook 4-2: 5.0-5.5 mm. These are starting points; exact fit depends on hook wire gauge and bead brand tolerances.

Slotted vs Round Beads

Slotted tungsten beads have a notch cut into the hole, allowing the bead to sit closer to the hook eye on jig-style hooks (Hanak, Dohiku style). Round beads are standard for straight-eye hooks. Slotted beads on jig hooks produce a keel effect that reduces snags significantly.