SpoolCalc Support
SpoolCalc · version 1.0 · last updated 15 August 2026
SpoolCalc answers one question: your reel is rated for one line diameter, but you want to spool a different one — how much will fit? And underneath that, how much backing goes under a braid top-shot?
Getting started
- Find the rating printed on your reel or in its manual, for example
0.30 mm / 200 m. - Type that into Reel rating.
- Type the diameter of the line you actually want to spool into New line.
- Read the estimate. It updates as you type — there is no calculate button.
Common questions
Why does it ask for diameter instead of pound test?
Because breaking strain tells you almost nothing about thickness. Two lines both sold as 30 lb can differ by half a millimetre, especially comparing braid with mono, and it is thickness that fills a spool. Line diameter is printed on virtually every spool and box; use that figure. Using pound test would give you a confident answer that happens to be wrong.
Why is the answer only an estimate?
The maths behind it is exact: line takes up space in proportion to its cross-section, and cross-section grows with the square of the diameter, so
L₂ = L₁ × (D₁² ÷ D₂²)
A real spool is messier than the model. Braid flattens and packs down under tension, so it usually takes less room than predicted. Real diameters drift from stated ones. Tapered and long-cast spools do not fill evenly. Line wound tight takes less space than line wound loose by hand. Expect the estimate to be close, not exact, and leave a couple of millimetres below the spool lip.
How does the backing calculator work?
It takes the spool's total volume, subtracts the volume your braid top-shot will occupy, and converts what is left into a length of backing:
backing = (L₁·D₁² − Ltop·Dtop²) ÷ Dbacking²
Wind the backing on first, join your braid to it, then fill the rest.
It says my braid "won't fit on its own". What does that mean?
The braid you asked for already needs more room than the whole spool has, so there is no space left for backing. The app tells you roughly how much of that braid the reel will actually hold. Either shorten the top-shot or choose a thinner braid.
Can I work in inches and yards?
Yes. Switch between metric and imperial in Settings → Units, or from the menu in the top right of either calculator. Everything you have saved is stored independently of that setting, so switching never alters your reels — the same reel just displays in different units.
Why does the app want a decimal comma or point?
It accepts both, whichever your keyboard shows. 0,18 and 0.18
are read the same way, so pasting a figure from a foreign tackle site works.
Does it work offline?
Completely. There is no server and no account. The only thing that needs a connection is buying or restoring SpoolCalc Pro, and once bought it keeps working with no signal.
SpoolCalc Pro
What do I get?
- the backing calculator;
- unlimited saved reels — the free version keeps two;
- calculation history, so you can re-run anything you worked out before;
- saved favourite setups, and future advanced calculators as they ship.
It is a one-time purchase, not a subscription. The line capacity calculator, unit switching and the explanation of the result stay free.
I bought Pro but a new phone shows it locked
Open Settings → Restore Purchases while signed in with the same Apple Account you bought it on. It restores free of charge.
I want a refund
Refunds are handled by Apple, not by the app. Use reportaproblem.apple.com. If something is broken, do also tell me — it may be fixable.
Reporting a bug
Use Settings → Report a Bug. The most useful reports say what you typed, what you expected, and what appeared instead. If a result looked wrong, include the exact numbers from all the fields.
Nothing is sent automatically. The app opens a draft in your own mail app and you decide whether to send it.
Contact
jakub.dwojak+spoolcalc@hotmail.com
SpoolCalc is made by one person, so replies are not instant — but every message is read.