This calculator determines the correct exposure time when enlarging or resizing black-and-white prints in the darkroom, so your prints maintain the same tonal balance as you change sizes.

Whether you’re scaling a print from an 8×10 to a 16×20 or reducing a larger print down to a smaller size, it helps keep exposure consistent instead of starting from scratch.

It works by applying either the standard linear-dimension-squared method for most print size changes, or a more advanced bellows factor formula when precision matters — especially for larger enlargements or significant magnification changes.

Only if you plan to use Precision Mode. Standard Mode works using just your original print width and the new print width you want to enlarge or reduce to.

If you enable Precision Mode, you’ll also enter your negative width (in inches or centimeters).

This allows the calculator to account for bellows factor, giving you a more accurate exposure time — especially when making larger enlargements or significant size changes.

Yes. You can use inches, centimeters, or millimeters — just be sure to use the same unit consistently for all measurements.

The calculations are based on ratios, so the units cancel out as long as everything is entered in the same measurement system.

Try to measure and enter the same linear dimension (either width or height) from both your original print and your new print. As long as you’re consistent, the calculator will give you a reliable exposure adjustment.

If you change both width and height significantly — meaning the aspect ratio changes — the result may be slightly less precise. In those cases, treat the calculated exposure as a strong starting point and fine-tune as needed at the enlarger.

Yes. The calculator works with all common film formats, including 35mm, medium format (120), and large format films such as 4×5 and 8×10. A reference table is included to help you quickly find typical negative widths if you’re unsure of the exact measurement.

As long as you enter consistent measurements, the calculator adapts to your format and printing workflow.

Yes. The calculator works with fiber-based papers and variable contrast filters as long as your enlarger aperture remains the same. Because the exposure adjustment is based on print size and magnification, the calculated time stays consistent regardless of paper type or contrast filter choice.

You can continue working with your preferred paper and filters without changing how you use the tool.

A few real-world factors can still affect exposure. The most common are changing the enlarger aperture without realizing it, inaccuracies in lens-to-paper distance, or uneven light falloff from the enlarger.

Paper age, developer activity, and processing consistency can also play a role.

This calculator is designed to get you very close by accurately accounting for print scaling and magnification. In practice, it provides a reliable starting point — and a quick confirmation test strip can help you fine-tune the final exposure when needed.

Even with accurate exposure calculations, a quick test strip can be helpful to confirm final exposure, especially when changing papers, developers, or contrast settings.

The calculator is designed to reduce the number of test strips needed while serving as an essential tool within a solid, consistent darkroom workflow.

Changing print size primarily affects exposure, not contrast.

However, large enlargements can sometimes reveal contrast or tonal differences due to paper response, light falloff, or lens characteristics. In those cases, minor contrast adjustments may still be needed after exposure is set.

This tool was created by Matthew Koller as part of the Unlock The Darkroom system and darkroom printing community.

It was built to help photographers spend less time guessing with test strips and more time making consistent, high-quality black-and-white prints in the darkroom.