1. Commodity data
When a 10-digit code is entered, BorderMath requests the UK Trade Tariff v2 commodity endpoint. If an origin country is selected, it is passed as a geographical-area filter so measures not relevant to that area can be removed by the official service.
2. Simplified customs value
For planning, the calculator adds goods value + freight + insurance. Customs valuation law can require adjustments or another valuation method, so this is labelled as an estimate.
3. Duty
Duty = simplified customs value × the percentage base-duty rate resolved in the calculator. When an origin is selected, BorderMath asks the official tariff service to filter the import measures for that geographical area. If one simple origin-specific preferential percentage rate is returned, the estimator can use it as an explicit eligibility-dependent assumption; if no simple preference is identified, it can use the published standard UK Global Tariff percentage rate. Multiple, conditional and quota treatments are not silently applied. Specific, compound, seasonal or otherwise non-simple duties are deliberately blocked rather than being reduced to the percentage portion of the tariff. Additional duties and trade remedies are surfaced separately rather than being treated as the base-duty rate.
4. Import VAT
BorderMath also inspects VAT import measures returned with the official commodity record. A clear percentage rate is used directly. Where HMRC identifies a standard, reduced or zero VAT treatment without exposing one clean numeric percentage, BorderMath maps that treatment to the current UK rate. If the response remains machine-ambiguous, the calculator continues with the current 20% UK standard rate as an explicit planning assumption rather than blocking the landed-cost workflow. Only genuinely conflicting VAT treatments require a choice. Estimated VAT value = simplified customs value + duty + other import costs entered by the user. Import VAT = estimated VAT value × VAT rate.
5. Low-value consignments
For Great Britain, the calculator currently applies the published £135 Customs Duty relief to non-excise goods using the goods value entered, not freight or insurance. Excise goods are excluded from this relief. VAT on consignments worth £135 or less is often accounted for at the point of sale, so users must avoid adding VAT twice if it is already included in the purchase price. The government announced in July 2026 that the £135 duty relief will be removed, with the operative date to be appointed by regulations by October 2028 at the latest. BorderMath models the current rule, not the future regime.
6. Landed cost
Total landed cost = goods + freight + insurance + duty + import VAT + other import costs.
Data freshness
Tariff API responses are cached briefly at the Cloudflare edge to reduce upstream load. The cache is deliberately short because tariff measures can change.