Understanding the UK CMS child maintenance formula

The situation

Child maintenance is one of the most contested areas of co-parenting — and one of the least understood. The CMS formula isn't complicated, but it's rarely explained clearly. This is our attempt to fix that.

How the CMS calculates payments

The CMS uses the paying parent's gross weekly income as the starting point.

• Up to £100/week → flat rate of £7/week
• £100–£200/week → 17% (1 child), 25% (2 children), 31% (3+)
• £200–£800/week → 12% (1 child), 16% (2 children), 19% (3+)
• £800–£3,000/week → 9% (1 child), 12% (2 children), 15% (3+)
• Over £3,000/week → additional calculation applies

Shared care reductions

If the paying parent has the child overnight, the payment is reduced: - 52–103 nights/year → 1/7 reduction - 104–155 nights/year → 2/7 reduction - 156–174 nights/year → 3/7 reduction - 175+ nights/year → flat rate only

This is why the custody schedule matters financially — not just emotionally.

What you can negotiate

Before going through the CMS, parents can agree a "family-based arrangement" — an informal agreement on amount and method. This avoids CMS fees (the paying parent is charged 20% on top; the receiving parent loses 4%). A family-based arrangement works best when both parents trust each other to stick to it, the amount is fair and documented, and there's a clear process for reviewing it annually.

What the CMS won't cover

The CMS only covers basic maintenance. It doesn't cover school uniforms and trips, childcare costs, medical expenses, or extracurricular activities. These need to be agreed separately — ideally in writing.

Where Co|Pare helps

Co|Pare's finance section lets you log shared expenses, attach receipts, and track what's been paid. It creates a clear, timestamped record that both parents can reference — and that can support any future CMS or court process.

The bottom line

Understanding the formula puts you in a stronger position — whether you're negotiating directly or going through the CMS. Knowledge reduces conflict. Always.