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
For an ad-valorem tariff, duty = simplified customs value × the percentage base-duty rate resolved in the calculator. BorderMath can also calculate supported weight-based duties from the shipment weight, including simple specific rates such as £18 per 100 kg and simple additive compound rates such as 10% + £93.10 per 100 kg. When an origin is selected, BorderMath asks the official tariff service to filter the import measures for that geographical area. If one unambiguous origin-specific preferential rate or supported formula is returned, the estimator can use it as an explicit eligibility-dependent assumption; otherwise it falls back to the applicable standard tariff treatment where that treatment is machine-calculable. MIN/MAX formulas, specialised measurement qualifiers, litres/items, conditional or quota treatments and other unsupported structures are not guessed. 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
For ordinary non-excise goods, total landed cost = goods + freight + insurance + duty + import VAT + other import costs. Where excise treatment is detected or selected, BorderMath labels the result as a partial landed-cost estimate because Excise Duty is not included and can also affect the VAT base.
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.
How BorderMath treats VAT-base and post-import costs
BorderMath separates qualifying incidental expenses that belong in the import VAT value from optional post-import business costs. The VAT-base field is intended for costs such as customs clearance, handling, storage and known onward transport that are part of the import VAT valuation. Post-import costs are added to the landed-cost estimate after VAT and are not used to calculate import VAT.
When BorderMath withholds a final total
If the official tariff data returns a potential trade-remedy/additional-duty measure or the user confirms excise-liable goods, BorderMath shows a clearly labelled partial subtotal instead of a final landed cost. This is because those duties can alter both the customs charges and the import VAT value. BorderMath also rejects invalid negative values, non-whole quantities and VAT percentages outside 0–100% rather than producing a numeric result.