Bag Calculator

How to use this bag fabric calculator

What this calculator does

This calculator converts a desired finished bag size into the rectangular fabric dimensions used by this simple open-top bag model. It uses the finished bag Width (w), Height (h), and Depth (d), then adds the model's fixed cutting allowance. When Box corners? is set to Yes, it also reports the corner distance associated with the selected depth. The result is a planning estimate for the main fabric rectangle, not a full sewing pattern: it does not add handles, pockets, linings, closures, interfacing, directional-print matching, shrinkage, or extra design ease.

When to use it

Use it when you are sizing a grocery tote, work tote, picnic bag, simple fabric basket, or another open-top rectangular bag. It is especially useful when you know the finished dimensions you want but need a quick cut-size estimate, when you are comparing several bag proportions before cutting fabric, or when you want to convert a project between metric and U.S. customary units. If your project uses a specialized commercial pattern, follow that pattern's allowances instead of replacing them with this estimate.

How to calculate

1. The calculator opens with a ready-to-use demonstration: 32 cm wide, 44 cm high, 18 cm deep, with box corners. Its Excel workbook is already built and validated, so Download Excel is available immediately.
2. Replace Width (w), Height (h), and Depth (d) with your desired finished dimensions. Enter positive decimal numbers using a period as the decimal separator; standard comma thousands grouping is accepted.
3. Choose the Unit. Switching among centimeters, millimeters, inches, and feet converts the current dimensions rather than merely relabeling them. The underlying calculation keeps full precision. NIST's published inch-to-metric conversion factors document the exact 1 inch = 2.54 cm relationship used here.
4. Set Box corners? to Yes if you want the depth-derived corner measurement. Set it to No if you only need the fabric rectangle.
5. Read Fabric width, Fabric height, and the combined Fabric cut size. If box corners are enabled, also read Box corner. Use Download Excel to save the current inputs and typed results in an XLSX workbook.
6. Reset clears the demonstration and all entered dimensions, returns the unit and corner choice to their neutral reset settings, and removes calculated results. Because the cleared state is incomplete, Excel export is disabled until you enter a complete valid set of dimensions again.

Input guide

Width (w) is required and represents the finished bag width. Enter a positive number in the selected Unit, such as 32 cm. A larger width increases fabric width by twice the width change because the rectangle has material for both sides of the bag. Do not enter the fabric width you already have; this field is the finished bag dimension. Height (h) is required and is the finished vertical bag height, for example 44 cm. Increasing height increases fabric height one-for-one. Depth (d) is required and describes the base depth, for example 18 cm. Increasing depth raises fabric width by twice the depth change, raises fabric height by half the depth change, and increases the box-corner measurement by half the depth change. Zero and negative dimensions are not valid for this model.

Unit is required and applies to all three dimensions and outputs. The accepted choices are centimeters, millimeters, inches, and feet. The inputs accept plain positive decimals such as 12.5 and conventionally grouped values such as 1,250.5; decimal-comma input such as 12,5 is rejected rather than silently reinterpreted. Box corners? is required as a Yes/No choice. Yes reveals the calculated corner distance; No keeps the same fabric-size calculation but labels the box-corner output as not requested.

Output guide

Fabric width is the calculated horizontal cut dimension. It equals twice the finished bag width plus twice the finished depth plus a fixed 1-inch total cutting allowance. Fabric height is the vertical cut dimension: bag height plus half the depth plus the same 1-inch total allowance. Both are estimates displayed in the active Unit and are driven by the finished dimensions. Box corner is exactly half of Depth (d) when requested; it describes the distance used to form the base corner in this model. A larger value produces a deeper base. The combined Fabric cut size simply presents Fabric width × Fabric height together; it is not an additional formula.

Worked example

The opening example uses Width = 32 cm, Height = 44 cm, and Depth = 18 cm. The fixed 1-inch allowance equals 2.54 cm. Fabric width is therefore 2 × 32 + 2 × 18 + 2.54 = 102.54 cm. Fabric height is 44 + 18 ÷ 2 + 2.54 = 55.54 cm. With box corners enabled, the corner size is 18 ÷ 2 = 9 cm. Those values match the first rendered results and the initial workbook checkpoints.

Learn more

The calculator's allowance is a fixed model assumption, while real projects may specify different seam allowances. Utah State University Extension's sewing instructions provide concrete examples of how a seam allowance is stated and applied in a sewing project. If your fabric, pattern, or construction method specifies a different allowance, treat that specification as authoritative for the actual cut.