CarpAnswers

Carp fishing tools

Reel line capacity & backing calculator

Estimate the mainline capacity of a carp reel and the backing needed beneath your chosen length of line.

Fill the spool with less guesswork

Enter the figures from your reel and line

The calculator treats the spool as a fixed line volume. Because line cross-sectional area changes with the square of its diameter, a small diameter change can make a noticeable difference to the estimated capacity.

Spool and line details
Example: 0.35mm on the reel or manufacturer specification.
The length paired with that printed diameter.
Use actual diameter where the maker publishes it.
The usable top section you want on the reel.
Backing can be a different diameter from the main line.

Estimated result

Your spool plan

Estimated backing to add first136musing 0.3mm backing
Estimated capacity at 0.35mm
400m
Spool volume used by mainline
75%
Mainline requested
300m
Formula used

Capacity index = printed length × printed diameter²

Backing = (capacity index − mainline length × mainline diameter²) ÷ backing diameter²

(400 × 0.35² − 300 × 0.35²) ÷ 0.3² = 136m