Measurement Price Calculator for WooCommerce

Custom Formulas

The custom formula builder lets you create any measurement calculation using variables, math operators, and functions. If the built-in calculator types don't fit your product, a custom formula will.

Overview

Custom formulas work by:

You define variables (e.g., width, height, radius).
You write a formula using those variables (e.g., width + (2 * height)).
Customers enter values for each variable on the product page.
The plugin evaluates the formula and calculates the price.

Setting up a custom formula

Go to the Measurement tab in any product.
Select Custom Formula as the calculator type.
Set the Pricing Unit (e.g., "units", "sq ft", or any label that makes sense).

Adding variables

In the Custom Variables section:

Click Add Variable.
Enter a Variable Name (e.g., width). Use lowercase letters and underscores only.
Set the Label (e.g., "Width"). This is what customers see.
Set the Unit (e.g., "ft", "in", "cm"). Displayed next to the input.
Optionally set Min, Max, and Step constraints.
Choose an Input Type: Free, Dropdown, or Hybrid.
Repeat for each variable your formula needs.

Writing the formula

In the Formula field, write your expression using:

Variable names you defined
Math operators: +, -, *, /
Parentheses for grouping: (, )
Math functions: min(), max(), ceil(), floor(), round(), sqrt(), abs()

Example formulas:

Product	Formula
Garage door seal	`width + (2 * height)`
Circular area	`3.14159 * radius * radius`
Box volume with walls	`(length + 2) * (width + 2) * height`
Rounded-up tile count	`ceil(area / tile_size)`
Minimum of two values	`min(length, width) * price_factor`

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Supported operators

Operator	Description	Example
`+`	Addition	`length + width`
`-`	Subtraction	`total - discount`
`*`	Multiplication	`length * width`
`/`	Division	`area / coverage`
`()`	Grouping	`(length + width) * 2`

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Standard math precedence applies: multiplication and division before addition and subtraction. Use parentheses to control order.

Supported functions

Function	Description	Example
`min(a, b)`	Returns the smaller value	`min(length, width)`
`max(a, b)`	Returns the larger value	`max(length, 10)`
`ceil(x)`	Rounds up to the nearest integer	`ceil(area / 4)`
`floor(x)`	Rounds down to the nearest integer	`floor(volume)`
`round(x)`	Rounds to the nearest integer	`round(weight * 2.2)`
`sqrt(x)`	Square root	`sqrt(area)`
`abs(x)`	Absolute value	`abs(length - width)`

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Functions can be nested: ceil(sqrt(area) * 2)

Live test panel

The formula builder includes a test panel to preview calculations before publishing.

After defining variables and writing a formula, the test panel appears.
Enter sample values for each variable.
Click Calculate to see the result.
If the formula has errors, a red error message appears instead.

The test panel helps you catch mistakes before customers see the product.

Common test panel errors

Error	Cause	Fix
Undefined variable	Formula references a variable not in the table	Add the variable or fix the name
Division by zero	A variable is zero and used as a divisor	Add a min value constraint > 0
Invalid expression	Syntax error in the formula	Check for missing operators or parentheses

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Input types

Each variable can use one of three input types:

Free input

A standard number field. Customers type any value within the min/max range.

Best for: Measurements that vary widely (room dimensions, fabric lengths).

Dropdown input

A select menu with predefined values. Customers pick from your list.

How to configure:

Set Input Type to Dropdown.
In the Options column, enter comma-separated values (e.g., 4, 6, 8, 10, 12).

Best for: Standard sizes (sheet sizes, pipe diameters, common lengths).

Hybrid input

A dropdown with a "Custom..." option that reveals a free input field.

How to configure:

Set Input Type to Hybrid.
In the Options column, enter your preset values.
When a customer selects "Custom...", a number input appears.

Best for: Products with common sizes but custom sizing available (window blinds, countertops).

Formula validation

The plugin validates formulas in two places:

Admin (JavaScript) - Validates as you type. Green border means valid, red border means error, yellow border means a referenced variable is missing.
Server-side (PHP) - Validates again when the cart calculates the price. This prevents manipulation of client-side values.

Using numbers in formulas

You can use numeric constants alongside variables:

3.14159 * radius * radius
width + 2.5
ceil(area / 32)

Decimal numbers and integers are both supported.

Example: garage door seal

A garage door seal needs material for the width plus both sides:

Variables:

Name	Label	Unit	Min	Max
`width`	Door Width	ft	6	20
`height`	Door Height	ft	6	12

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Formula: width + (2 * height)

Pricing unit: ft

Result: A 16 ft wide, 7 ft tall door → 16 + (2 × 7) = 30 ft of seal material.

Example: circular table cover

A circular table cover priced by area:

Variables:

Name	Label	Unit	Min	Max
`diameter`	Table Diameter	in	24	96

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Formula: 3.14159 * (diameter / 2) * (diameter / 2)

Pricing unit: sq in

Result: A 48 in table → 3.14159 × 24 × 24 = 1,809.56 sq in.

Example: tile count

Calculate the number of tile boxes needed:

Variables:

Name	Label	Unit	Min	Max
`length`	Room Length	ft	1	100
`width`	Room Width	ft	1	100

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Formula: ceil((length * width) / 25)

This divides the room area by 25 sq ft (the coverage per box) and rounds up.

Pricing unit: boxes

Result: A 12 ft × 15 ft room = 180 sq ft → ceil(180 / 25) = ceil(7.2) = 8 boxes.

Best practices

Keep formulas simple. If a built-in type works, use it instead.
Set min/max constraints on every variable to prevent extreme values.
Test with edge cases. Try minimum values, maximum values, and zero.
Use descriptive variable names. door_width is clearer than x.
Use the test panel before publishing to verify your formula.
Document your formula in the product description so customers understand the calculation.

Troubleshooting

Formula shows "Invalid expression"

Check for missing operators between values (e.g., width height should be width * height).
Check for unmatched parentheses.
Make sure function names are spelled correctly.

Result is different from expected

Check operator precedence. Use parentheses to be explicit.
Verify variable names match exactly (case-sensitive).
Test with known values in the test panel.

Variable not recognized

Check spelling matches exactly between the variable name and the formula.
Variable names are case-sensitive: Width is different from width.
Remove any spaces from variable names.

Calculator Types

Pricing Tables