Frequently Asked Questions

HMRC PAYE Tax Table Routines prescribe per-period rounding (weekly / monthly) that doesn't equal a flat division of the annual figure. Switch the calculation basis to Annual (Self Assessment view) for the year-end total or keep monthly / weekly to match the per-payslip number.

Tax codes encode your personal allowance and any special treatment.

• 1257L — standard code, full £12,570 personal allowance.
• K-codes — negative allowance (extra taxable amount, e.g. taxable benefits).
• BR / D0 / D1 — flat-rate codes (basic / higher / additional rate on every penny). Common for second jobs.
• S- prefix — Scottish taxpayer (uses SRIT bands).
• C- prefix — Welsh taxpayer (currently same as rUK numerically).
• 1257M / 1257N — marriage allowance transferred in / out.
• NT — no tax (rare; usually applies to specific reliefs).
• W1 / M1 / X markers — force non-cumulative basis.

Roughly: outside-IR35 contracts → Limited Company is more tax-efficient because you can split income between salary and dividends. Inside-IR35 → an umbrella is simpler than running a Ltd Co you can't optimise. Use the IR35 calculator to see the actual £ delta for your specific day rate.

Yes. Tick the Scotland box, or use a tax code with an S prefix (e.g. S1257L). The calculator uses Scottish income tax bands (SRIT). Note: dividend tax is not devolved — Scottish taxpayers still use rUK bands for dividends, even though their salary uses Scottish bands.

All three put money in your pension; they differ in when the tax break is given.

• Relief at Source — contribution from net pay; provider adds basic-rate relief; higher-rate via Self Assessment. Common with NEST.
• Net Pay arrangement — contribution from gross before income tax; NI on full gross. Common with occupational schemes.
• Salary Sacrifice — agreed lower salary; both income tax and NI on the reduced figure. Best for take-home; reduces statutory pay base (e.g. SMP).

It's an estimate. The math follows the HMRC spec to the penny for the cases listed in the README conformance table, but your actual liability depends on data we don't have (other employments, benefits in kind, capital gains, etc.). Don't file a Self Assessment off this; consult HMRC's guidance or a qualified UK accountant for final figures. See Terms of Use for the full disclaimer.

Yes — public, free, throttled at 60 requests / minute per IP. POST your inputs to /api/calculate/<mode>/ where mode is one of paye, limited_co, umbrella, ir35 or sole_trader. See the API documentation for request shapes and curl examples.

Yes — an embeddable <iframe> widget is live at /business/, free for any domain or volume. The calculation engine is also open source under the AGPL-3.0 licence — fork it or call the public API and build your own UI.