Guide
The cable reel capacity formula
The standard cable reel capacity formula is length in feet = 0.262 × (H + D) × H × T ÷ d², where H is the winding depth, D the drum diameter, T the traverse and d the cable diameter, all in inches.
The formula gives an upper estimate that assumes cable packs perfectly. Real cable winds less neatly, so the useful habit is to calculate, then allow a margin.
The terms
- F, flange diameter
- Edge to edge across the flange.
- D, drum diameter
- Across the barrel the cable winds on.
- T, traverse
- Inside width between the flanges.
- c, clearance
- Space left below the flange rim, usually about 2 inches, so the outer layer is protected.
- H, winding depth
- (F − D) ÷ 2 − c.
- d, cable diameter
- Outside diameter of the cable, jacket included.
A worked example
Take a 48-inch reel with a 24-inch drum and a 28-inch traverse, and a cable 1 inch across. Winding depth H = (48 − 24) ÷ 2 − 2 = 10 inches. Then length = 0.262 × (10 + 24) × 10 × 28 ÷ 1² = 0.262 × 34 × 280, which is about 2,494 feet.
Counting layers instead gives a closer figure: 10 layers of 28 turns each, with each layer's turn longer than the last. That gives about 2,490 feet for this reel, almost the same here; with thicker cable the two methods drift further apart, and the layered count is the one to trust.
Where 0.262 comes from
It is π ÷ 12, about 0.2618. Each turn of cable is a circle whose average diameter is H + D; multiplying by π gives its length in inches, and dividing by 12 converts to feet. H × T ÷ d² is the number of turns that fit in the winding area.
Margins to allow
- Cable that is stiff or has an oval cross-section packs less tightly.
- Uneven winding on a job site loses capacity fast.
- Large cable rounds badly to whole layers; drop to the layered figure.
- Leave the full clearance. Cable proud of the flange rim gets crushed.
Using it backwards
To find the reel for a length, rearrange for the drum and traverse you have, or use the calculator, which checks each common size against the length you need.
Why outer layers hold more
Every layer wraps a slightly bigger circle than the one beneath, so the outermost turns are the longest. On a reel with a small drum and deep flanges, the top third of the winding depth can hold close to half the cable. That is also why a reel that looks half-full by depth is usually well under half-full by length: judge remaining cable by counting layers, never by eye. Crews pulling feeders through conduit in a Loop basement or along a Joliet warehouse wall find this out quickly when a reel runs short mid-pull.
Common inputs people get wrong
- Using the conductor size instead of the overall cable diameter with insulation and jacket.
- Forgetting clearance, which overstates capacity by a layer or two.
- Using the outside width instead of the traverse.
- Mixing feet and inches: every input is in inches, and the answer comes out in feet.
- Rounding the cable diameter down: a quarter-inch error on thin cable changes the answer by hundreds of feet.
- Typing a flange radius where the formula expects the full diameter.
Metric version
In millimetres and metres, the same formula reads: length in metres = π × (H + D) × H × T ÷ (d² × 1000), with every dimension in millimetres. The structure is the same: average turn length times the number of turns that fit.
A second example
A 36-inch reel with an 18-inch drum and a 20-inch traverse, holding a cable 0.75 inches across: H = (36 − 18) ÷ 2 − 2 = 7 inches. Length = 0.262 × (7 + 18) × 7 × 20 ÷ 0.5625, which is about 1,630 feet. Nine full layers of 26 turns fit in that depth, which puts the layered count a little lower, so plan for roughly 1,500 feet in practice.
Questions
What is the reel capacity formula?
Length (ft) = 0.262 × (H + D) × H × T ÷ d², with H the winding depth, D the drum, T the traverse and d the cable diameter in inches.
Why is real capacity lower?
Cable does not pack perfectly, and stiff or large cable loses space at the ends of each layer.
Is there a calculator?
Yes, the reel capacity calculator works out both the formula and the layered figure.
Does the formula work for rope?
Yes, using the rope diameter. Rope packs more neatly than stiff cable, so real capacity sits closer to the formula.
Reel words on this page
- Flange
- The round side discs that hold the cable in. Flange diameter, edge to edge, is the size a reel is known by.
- Drum (barrel)
- The cylinder between the flanges that the cable winds around.
- Traverse
- The inside width between the flanges: how wide a layer of cable can be.
- Payoff
- The stand or trailer a reel turns on while cable is pulled off it.
More reel guides
- What empty reels are worth
Size, build and condition decide it. What gets paid for, and what does not.
- Do supply houses take reels back?
When the counter or the maker behind it takes reels, and what happens to the rest.
- Returnable vs one-way reels
Reading the tag, the stencils and the build to know whose reel it is.
- Which makers take reels back
Published maker return programs, their minimums and their condition rules.
- Reel sizes explained
Flange, drum and traverse, with a size chart from 24 to 96 inches.
- Measuring a reel
Four measurements, one tape and a minute per reel.
- Nailed vs bolted reels
How each is built, why bolted reels make more trips, and how each is handled.
- When a reel can't be reused
The defects that end a reel's working life, drawn and explained.
- Disposing of wooden reels
Four ways out for a wooden reel, with the dumpster last.
- Staging reels for pickup
Stand, chock, sort and leave room for the forks.
- Reel weights by size
Empty weights worked out from the wood, and what each means for loading.
Reels to clear or a size to find?
Tell us what you have or what you need. One work order covers pickup, buyback, recycling and supply.
Or write to info@chicagowoodenspools.com with the count, flange sizes and a photo of the stack.