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How to use math functions in price formulas

The price formula field type in WooCommerce Product Options allows store owners to create custom calculations for product prices based on information that the customer provides. Price formulas can be very simple or very complex!

This article provides details of how to use advanced mathematical formulas in your price formulas. The following information is just a starting point and you can use any of the mathematical operators documented in this JavaScript formula parser, which WooCommerce Product Options uses under the covers.

Available variables

You can use price formulas to calculate the final price of a product based on variables such as:

  • Numbers entered by the customer. These must come from number fields which are in the same product options group as the price formula field.
  • The base price of the current product [product_price].

Mathematical operators

The price formula field type supports common mathematical operators, such as +, -, * and /. In addition, it can parse all the mathematical functions included in the Javascript Math object.

A complete list of those functions can be found on the Javascript Math page of the MDN Web Docs. Those functions can be used by entering their name (without the Math. prefix itself) in the formula. Below, we will explore some practical examples to help you to apply this to different use cases.

How to limit prices

The functions min and max allow you to set a minimum or maximum price that can be charged. For example, you might be selling made-to-measure curtains (like in this demo product) and want to charge a minimum of $50 and a maximum of $500, regardless of the measurements entered.

You should use the max function to set a minimum price and the min function to set a maximum price. (We realise this is the opposite of what you might expect linguistically - this is how these mathematical functions work and is not specific to the WooCommerce Product Options plugin 🙃)

Example: max

Use this to set a minimum price for the price formula.

A store sells made-to-measure curtains. The product costs $50 per square meter. Customers are asked to enter the width and height they need – in cm – but they will incur a minimum charge of $50 regardless of the total size being ordered.

You can achieve this with the following formula:

max( 50, ( [width] / 100 ) * ( [height] / 100 ) * [product_price] )

In the formula above, the function will return [width] * [height] * [product_price] if it is more than 50. Otherwise, it will always return 50, no matter how small the values for [width] and [height] are.

For example, when width is 100 and height is 50, the formula is calculated as follows:

max( 50, ( 100 / 100 ) * ( 50 / 100 ) * 50 ) = max( 50, 1 * 0.5 * 50 ) = max( 50, 25 ) = 50

Example: min

Use this to set a maximum price for the price formula.

A store sells guided tours of archeological sites. Tourists pay a fee of $10 per ticket but larger groups can get a deal because the tour is capped at a maximum charge of $400 (corresponding to 40 people) but can accommodate up to 50 people (so every person in that group exceeding 40 people gets to participate for free).

You can achieve this with the following formula:

min( 400, [tickets] * [product_price] )

In the formula above, the function will return [tickets] * [product_price] if it is less than 400. Otherwise, it will always return 400, no matter how large the value for [tickets] is.

For example, when the number of tickets is 48, the formula is calculated as follows:

min( 400, 48 * 10 ) = min( 400, 480 ) = 400

How to round prices

You can round the value of a calculated price by using function among round, trunc, floor and ceil, with the following specifications:

  • round approximates the value to the closest integer (e.g round( 3.37 ) returns 3; round( 5.98 ) returns 6).
  • floor returns the closest integer smaller than the input value (e.g floor( 3.37 ) returns 3; floor( 5.98 ) returns 5).
  • trunc truncates the decimal portion of a number (e.g. trunc( 3.37 ) returns 3; floor( 5.98 ) returns 5). With positive numbers, trunc and floor both return the same value.
  • ceil returns the closest integer greater than the input value (e.g floor( 3.37 ) returns 4; floor( 5.98 ) returns 6).

Example: ceil

A store sells pet food. Dog food is $28.15 per kg. Customers can enter the required weight with 0.1 kg precision. The shop owner wants prices to be calculated to the whole dollar, approximating by excess. The appropriate formula is:

ceil( [weight] * [product_price] )

For example, if 2.5 is entered as weight, the formula is calculated as follows:

ceil( 2.5 * 28.15 ) = ceil( 70.375 ) = 71

Example: round

A website provides housekeeping services and allows customers to book cleaners in 1-hour slots. The cost for 1 hour is $19.85 and the site owner wants to round the price calculation in $5 increments so that $19.85 returns $20 and $357.30 returns $355. The appropriate formula is:

5 * round( [hours] * [product_price] / 5 )

For example, if a customer requests 18 hours, the formula is calculated as follows:

5 * round( 18 * 19.85 / 5 ) = 5 * round( 71.46 ) = 5 * 71 = 355

Logical or conditional clauses

Price formulas are limited to mathematical operations and do not understand logical structures. Therefore, expressions like if ( [width] < 200 ) then [product_price] else 1.25 * [product_price] cannot be used in a price formula. Instead, you can use mathematical functions and operators to transform logical clauses into numbers.

Equivalence between logical and mathematical constants and operators

The basic equivalencies between logic and maths are:

  • true and false are equivalent to 1 and 0, respectively
  • a AND b is equivalent to a multiplication a * b
  • a OR b is equivalent to an addition a + b
  • a > b can be written as max( 0, sign( a - b ) )
  • a < b can be written as max( 0, sign( b - a ) )

Let's see how this can help us write conditional structures in the context of price formulas. Please consider the following example. A store sells shipping services and doubles its rates for oversized items. Users are invited to enter the Width, Height and Depth of the package they need to ship. The store considers a package oversized if the sum of the three lengths is more than 100 centimeters. In a regular coding language, one would write:

if [Width] + [Height] + [Depth] > 100 then 2 * [product_price] else [product_price]

but we cannot write this in a price formula. What we would write is:

[product_price] * (1 + max( 0, sign( [Width] + [Height] + [Depth] - 100 ) ) )

The function sign returns 1, -1 or 0 depending on the passed parameter being positive, negative or zero. Since max forces the result to be greater than 0, what the function does is return 1 if [Width] + [Height] + [Depth] is greater than 100 and 0 otherwise.

For example, if a customer wants to ship a package that is 45 cm wide, 40 cm high and 20 cm deep, the formula is calculated as follows:

[product_price] * ( 1 + max( 0, sign( 45 + 40 + 20 - 100 ) ) ) =
[product_price] * ( 1 + max( 0, sign( 5 ) ) ) =
[product_price] * ( 1 + max( 0, 1 ) ) =
[product_price] * ( 1 + 1 ) =
[product_price] * 2

How to use AND/OR logic in a price formula

As described above, you must use mathematical operators in order to add AND/OR logic. Specifically:

  • Use addition (the + sign) for OR logic.
  • Use multiplication (the * sign) for AND logic.

For example: [Sides.Front.checked] * [Sides.Back.checked] * 10 translates into "If BOTH front AND back sides are checked then add 10 to the formula", whereas ( [Sides.Front.checked] + [Sides.Front.checked] ) * 10 / [Sides.count] translates into "If EITHER the front OR the back side are checked add 10 to the formula".

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