retail.price-per-unit

export function unitPrice(price: Money, quantity: string, unit: string, per: UnitPriceBasis): Money

export type UnitPriceBasis = "kg" | "100g" | "10g" | "L" | "100mL" | "10mL" | "75cL" | "m" | "m2" | "m3" | "item";

import { unitPrice } from "#fune/retail.price-per-unit@^1";

import { type Money, money } from "./money_amount.ts";  ← from money.amount ^1.0.0 · built alongside by fune
import { convertUnits } from "./units_convert.ts";  ← from units.convert ^1.0.0 · built alongside by fune
import { type UnitPriceBasis } from "./retail_price_per_unit_types.ts";

// Each basis is a units.convert symbol and how many of it the price is for.
const BASES: Record<string, [string, number]> = {
  kg: ["kg", 1],
  "100g": ["g", 100],
  "10g": ["g", 10],
  L: ["L", 1],
  "100mL": ["mL", 100],
  "10mL": ["mL", 10],
  "75cL": ["mL", 750],
  m: ["m", 1],
  m2: ["m2", 1],
  m3: ["m3", 1],
  item: ["item", 1],
};

const DECIMAL = /^[0-9]+(\.[0-9]+)?$/;

/**
 * The price of one `per` of the product, rounded half up to the minor unit.
 *
 * The quantity is converted exactly (units.convert, 12 decimal places), then
 * price x multiplier / quantity is done in integers, so the one rounding step
 * is the last one.
 */
export function unitPrice(price: Money, quantity: string, unit: string, per: UnitPriceBasis): Money {
  if (price.minor < 0) throw new RangeError(`price must not be negative, received ${price.minor}`);
  const basis = BASES[per];
  if (basis === undefined) throw new RangeError(`unknown unit price basis "${per}"`);
  const [target, multiplier] = basis;

  let converted: string;
  if (target === "item" || unit === "item") {
    if (target !== "item") throw new RangeError(`a quantity in items can only be priced per "item"`);
    if (unit !== "item") throw new RangeError(`per "item" needs the quantity in "item"`);
    if (!DECIMAL.test(quantity)) throw new RangeError(`quantity must be more than 0, received "${quantity}"`);
    converted = quantity;
  } else {
    converted = convertUnits(quantity, unit, target, 12);
  }
  if (converted.startsWith("-") || !/[1-9]/.test(converted)) {
    throw new RangeError(`quantity must be more than 0, received "${quantity}"`);
  }

  // converted = digits / 10^places, so price / converted = price x 10^places / digits.
  const [whole, fraction = ""] = converted.split(".");
  const digits = BigInt(whole + fraction);
  const numerator = BigInt(price.minor) * BigInt(multiplier) * 10n ** BigInt(fraction.length);
  let quotient = numerator / digits;
  if ((numerator % digits) * 2n >= digits) quotient += 1n;
  return money(Number(quotient), price.currency);
}

Install

fune build

With that line in your source, in a TypeScript project (language typescript in fune.project), fune build resolves it and its 2 dependencies, pins them in fune.lock, downloads only the TypeScript package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add retail.price-per-unit

The manifest, vectors and README with only the TypeScript implementation. Install it without the registry with fune add ./retail.price-per-unit-1.0.1-typescript.fune, or fetch it from a terminal with fune pull retail.price-per-unit@1.0.1:typescript.

The whole function, every language, is one file too: retail.price-per-unit-1.0.1.fune, 18,495 bytes, sha256 bb9baca1fabbefbfb0d311fb8f46242ae294c3a78f05a541398b361ce3e780e5. It installs into a project of any language.

// fune: before retail.price-per-unit

// fune: after retail.price-per-unit

// fune: replace money.amount in retail.price-per-unit
// fune: replace units.convert in retail.price-per-unit

// fune: step retail.price-per-unit after <n|label>

More from the author

The Price Marking Order 2004 (SI 2004/102) requires a unit price alongside the selling price for most goods sold loose or in pre-set quantities, and defines it as the final price, VAT included, per kilogram, litre, metre, square metre or cubic metre, or per item for goods sold by number.

- **Until 5 April 2026**, Schedule 1 of the Order let listed products use other units: per 10 g or 10 ml (herbs, spices, make-up), per 100 g or 100 ml (biscuits, coffee, soft drinks, sauces and many more), per 75 cl for wine, per 50 kg for coal. - **From 6 April 2026**, the Price Marking (Amendment) Order 2024 (SI 2024/1055, commencement moved from 1 October 2025 by SI 2025/592) omits Schedule 1. The unit price is per item for goods sold by number, per kilogram for goods marked only by weight, per litre for goods marked only by volume, and otherwise per kilogram, litre, metre, square metre or cubic metre. The deposit (for example a deposit return scheme charge) is excluded.

This function does not decide which unit a product needs; that depends on the product and on the date, and the caller's product data knows both. It takes the unit as `per` and does the arithmetic exactly. `100g`, `10g`, `100mL`, `10mL` and `75cL` stay available for labels printed under the old Schedule 1, for shelf labels outside Great Britain, and for voluntary extra prices.

## Arithmetic

The pack quantity is converted to the unit of `per` with `units.convert`, which is exact rational arithmetic, to 12 decimal places (so 8 oz is exactly 226.796185 g). The price is then divided by that quantity in exact integer arithmetic and rounded once, half up, to the minor unit: 6 rolls for £1.00 is 16.67p, shown as 17p. There is no floating point anywhere, so £4.00 per pound is 882p per kg in every language.

## Edge cases

- `unit` `"item"` goes with `per` `"item"` only, and the other way round. - A quantity of zero, or one so small it converts to 0 at 12 decimal places, is an error; so is a negative price. A free item's unit price is 0. - Mass and volume do not convert into each other: pricing 500 g per litre is an error from `units.convert`. Where the law lets a product be sold by weight or volume, pass whichever the pack is marked with.

## Sources

- The Price Marking Order 2004, SI 2004/102, article 1 (definition of "unit price") and Schedule 1, as made: https://www.legislation.gov.uk/uksi/2004/102/article/1/made and https://www.legislation.gov.uk/uksi/2004/102/schedule/1/made - The Price Marking (Amendment) Order 2024, SI 2024/1055 (new definition of "unit price", Schedule 1 omitted): https://www.legislation.gov.uk/uksi/2024/1055/made - The Price Marking (Amendment) Order 2025, SI 2025/592, article 2(2) (commencement moved to 6 April 2026): https://www.legislation.gov.uk/uksi/2025/592/body/made

1.0.1 fixes Python accepting a trailing newline in quantity (priced per item); adds tests.

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