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Some prices don’t come in tiers. A sign shop’s quote is often a curve: the first sign carries the setup, each further sign costs less, and every number depends on the sign’s size. You can approximate that with quantity breaks, but you need a row for every quantity, and a product the customer sizes themselves can’t be broken down by size and quantity at the same time. A Formula price rule calculates the price directly from the job:
You write the formula once, in the builder, and the configurator works the price out for whatever size and quantity the customer enters.
Formula pricing is available on Shopify, and on WooCommerce from plugin version 0.4.16.

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When to use a formula

Reach for a formula when:
  • your price list is already a spreadsheet formula,
  • the price depends on the size and the quantity together, as with custom-size signs, banners and panels,
  • each item has a minimum price, whatever its size, or
  • you price in size bands and want quantity to count as well.
For simpler cases the other tools are quicker to set up: a per-unit price with quantity breaks, a price matrix when your supplier quotes every combination, or area pricing at a flat rate per square metre.

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How a formula rule works

A formula is a price rule like any other, so it lives in the same places:
  • On a field (its Pricing tab): it applies while that field is shown. A formula on the size field is the usual choice.
  • On a choice (open the choice’s details, then Pricing): it applies while that choice is selected. Give each material its own formula when aluminium and foam board are priced differently.
The formula’s result is the rule’s whole charge for the order. Unlike a per-unit price, it is not multiplied by the quantity afterwards. Use qty in the formula wherever the quantity matters. Everything else in your option set still applies around it: the base price is added, a choice’s percentage (a rush surcharge, say) applies to the running total after it, and setup fees and the minimum price come last. To let the formula be the whole price, set the base price to 0, once per order.

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Your first formula: custom-size signs

This walk-through builds the sign price from the top of this page. Each sign size has three numbers, all worked out from the sign’s area in square feet:
  1. Add the size. Add a Dimensions field labelled Size, with inches as its unit. (The Posters template already has one, in centimetres.)
  2. Zero the base price. Under Product settings, set the base price to 0, charged Once per order.
  3. Add the rule. Select the Size field, open its Pricing tab, click + Add price rule, and change the rule’s type to Formula. A starter formula, 10 * area * qty, appears.
  4. Set the units. Size from is the field the formula reads (Size). Set w and h in to in and area in to ft². The builder picks these for you from the size field: inches go with square feet, centimetres with square metres.
  5. Add the steps. Click + Add step three times, and fill in the names and formulas from the table above: P1 = 25 + 6 * area, F = 4 + 3.5 * area, b = 8 + 2 * area.
  6. Write the price. In Price =, enter P1 + F * (qty - 1) + b * (1 - 0.25^(qty - 1)) / 0.75.
  7. Check it. Open Check prices, enter a size such as 24 × 18, and compare the table with your own price list (see Checking your prices).
  8. Publish.
For three sign sizes, that formula charges:
Steps are optional. You could write the whole price as one long formula, but naming the parts the way your price list does (first sign, further signs, premium) makes the formula readable months later, and an error points at the step it is in.

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Writing formulas

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Names you can use

  • Names don’t care about capitals: Qty, QTY and qty are the same.
  • w, h and area are 0 until the customer has entered a size. If the rule’s Size from is set to Uploaded file canvas, they come from the artwork’s own size instead of a dimensions field.

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Operators

Two things a spreadsheet lets you do that a formula doesn’t:
  • Write the multiplication. 2 * area, not 2area.
  • Bracket a negative power. Spreadsheets read -2^2 as 4, maths reads it as −4. To avoid surprises the builder asks you to write (-2)^2 or -(2^2). 2^-1 (a negative exponent) is fine as it is.

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Functions

band() works like a price-break table: it checks the limits from left to right and gives the value of the first band that x fits under (each limit is included in its band), or the last value when x is above them all. band(area, 2, 30, 6, 26, 22) is 30 up to 2 ft², 26 up to 6 ft², and 22 above that.

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From a spreadsheet

Pasting a formula from a spreadsheet or a document works: the builder turns × into * and − or – into - as you paste. Spreadsheet functions translate like this:

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Limits

  • Up to 10 steps, and up to 500 characters per step or price.
  • A step name starts with a letter and then uses letters, digits or _ (up to 32 characters). It can’t reuse a name the formula already has, such as area, qty or max.
  • Only letters, digits, spaces and _ . , + - * / ^ ( ) are allowed. There are no comparisons (<, >) and no if: use min, max and band instead. (WordPress would rewrite <, > and % when your option set is saved, so they are kept out of formulas on every platform.)
  • A formula rule has no volume breaks of its own. Put the breaks in the formula with band().
  • A formula can’t go on one option of a quantity per option field. Put it on the field itself, where qty is the total.

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Units

w and h in and area in are separate on purpose. A US sign shop measures in inches and prices by the square foot: with in and ft², a 24 × 18 in sign reads as w = 24, h = 18 and area = 3. The customer can enter the size in any unit the size field offers. The formula always sees it converted into your units. Sizes are converted exactly as you would expect on paper. A 12 in width is 12 in the formula, not 11.999…, so floor(w) and band() limits behave.

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Checking your prices

Check prices, under the formula, shows what the rule charges at one size for several quantities: the quantity field’s presets if it has some, otherwise 1, 2, 5, 10 and 25. This rule adds is the rule’s total; Each is that divided by the quantity. It uses the same calculation as your storefront. The table shows this rule only. Other rules (a material surcharge, a rush percentage, a setup fee, the minimum price) still apply on top, and the Preview pane on the right shows the whole product as your customers will see it.

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When the builder flags a formula

Mistakes show in red under the step or price they are in, as you type, and the option set can’t be published until they are fixed:

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When a formula has no price

Some formulas have no answer for some jobs: dividing by qty - 1 when one sign is ordered, or sqrt of a negative number. The configurator then tells the customer This size can’t be priced online. Please contact us. and won’t add the job to the cart. A job is never sold at a price of 0 because a formula broke. To keep every size priceable, give the size field a minimum and maximum width and height, and guard any division, for example 100 / max(qty - 1, 1).

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Recipes

A per-area price undercharges small items. This charges $12.99 per square foot, but never less than $30 a sign:
Ten 9 × 12 in signs (0.75 ft² each) cost $300 rather than $97.40.
$30 a sign up to 2 ft², $26 up to 6 ft², $22 above:
Round the area up before pricing it, so 2.25 ft² is billed as 3:
Hemming or edge banding is priced by the length around the item. With w and h in set to ft:
0.1% off per item ordered, capped at 30%:
A $35 setup and $0.80 per item, as one rule:
(A setup fee under Product settings does the same for the setup part, and is never scaled by percentages or multipliers.)
Four pages fit on one sheet, and a started sheet is a whole sheet. With a file upload that reports the page count:
Put a formula rule on each material choice instead of on the size field: only the selected material’s formula applies.

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Questions

Not yet: a formula reads the quantity, the size and the page count. For choices such as material or finish, put a formula (or a percentage) on each choice instead.
No. The configurator shows a preview, and the same formula runs again on our servers when the job is added to the cart. Your store charges the price our servers calculate and sign, as with every other price rule.
Check prices shows the formula rule on its own. The storefront adds the base price, other price rules on the selected choices, percentages, setup fees and the minimum price. Set the base price to 0 if the formula should be the whole price.
The hint reads quantity breaks, so a product priced only by a formula doesn’t show it. See the next price break.