CSA Calculator for Self Employed

Under Regulations 36 and 37 of the Child Support Maintenance Calculation Regulations 2012, self-employed parents (sole traders and partners) are assessed based on their taxable trading profits as declared to HMRC in their latest Self Assessment tax return. Specifically, the CMS pulls the figure from Box 31 of the SA302 Tax Calculation (or Box 28 of the self-employment supplementary pages).

Gross turnover is never used directly as the assessment base. Legitimate trading expenses deducted on the tax return naturally reduce assessable profit before child maintenance is calculated.

Any expense incurred wholly and exclusively for the purposes of trade lowers your taxable profit. This includes commercial vehicle costs, raw materials, office rent, professional indemnity insurance, accountancy fees, and capital allowances.

However, the CMS can scrutinize excessive capital allowances or personal drawings through the Variation Application process if the receiving parent alleges that profits were artificially depressed to reduce child support.

Under the statutory 25% change tolerance rule (Regulation 34), if your current trading profits have dropped by at least 25% compared to your historical SA302 return, you do not have to wait for your next annual tax return. You can submit interim management accounts, up-to-date sales ledgers, or business bank statements to request an immediate mid-year reassessment.