Exposure Calculator for Darkroom Printers
Built for analog photographers and students of Unlock the Darkroom, this calculator helps you keep exposures consistent when resizing your black and white prints.
Eliminates guesswork when changing print sizes — no more wasted paper or time
Works with all formats — from 35mm and medium format to large format negatives.
Includes both quick and precision modes, so you can choose speed or accuracy.
Built to support the Unlock the Darkroom system and help photographers print with confidence.
Darkroom Print Exposure Calculator
Use any consistent unit (inches or cm) — just keep it the same across all fields.
Standard Negative Widths
Use these as a reference for entering negative width in Precision Mode. Values shown are for image width (not full film strip size).
| Film Format | Width (in) | Width (cm) | Width (mm) |
|---|---|---|---|
| 35mm | 1.42 in | 3.6 cm | 36 mm |
| 120 (6×4.5) | 1.77 in | 4.5 cm | 45 mm |
| 120 (6×6) | 2.36 in | 6.0 cm | 60 mm |
| 120 (6×7) | 2.75 in | 7.0 cm | 70 mm |
| 120 (6×9) | 3.54 in | 9.0 cm | 90 mm |
| 4×5 | 5.0 in | 12.7 cm | 127 mm |
| 5×7 | 7.0 in | 17.8 cm | 178 mm |
| 8×10 | 10.0 in | 25.4 cm | 254 mm |
Calculate Perfect Exposure Times for Any Darkroom Print Size
Every darkroom printer knows the struggle of getting a perfect 8×10 print—only to blow the highlights on the 16×20.
This free darkroom print exposure calculator solves that problem by helping you calculate exactly how long to expose your paper when changing print sizes.
Whether you’re using the standard enlargement formula or diving deep into bellows factor math, this tool gives you reliable, repeatable exposure times—no test strips, no wasted paper.
Designed for film photographers around the world and students of Unlock the Darkroom.
How to Use This Tool
This calculator helps you adjust your exposure time when making larger or smaller black and white prints in the darkroom — without wasting paper or doing endless test strips.
Whether you're scaling up from an 8×10 to a 16×20 or shrinking a work print down to a contact sheet, this tool gives you an accurate exposure time based on print size.
🟢 Standard Mode (Quick & Reliable)
Enter your original exposure time — the time that gave you a perfect print.
Click Calculate, and you’ll get a new exposure time based on the standard “linear dimension squared” method. This works great for most enlargements and reductions.
🔵 Precision Mode (Bellows Factor Formula)
Check the “Use Precision Mode” box.
Enter your negative width (use the film format reference table if you’re not sure).
Enter your original and new print widths, just like in Standard Mode.
Click Calculate. This uses the bellows factor formula, which accounts for lens extension and magnification changes — ideal for large prints or ultra-precise exposure matching.
Enter your new print width — the size you want to make now.
How to Use This Tool
Unlike generic exposure calculators or outdated darkroom charts, this tool was built specifically for modern analog photographers who want precision, control, and consistency in their darkroom workflow. It doesn’t just spit out numbers — it’s designed to support the real-world process of making beautiful, repeatable black and white prints.
This calculator offers two modes: a quick solution for day-to-day printing, and a precision mode that accounts for bellows factor and magnification — perfect for large-scale fiber-based prints where tonal detail and exposure accuracy really matter.
Whether you're working with 35mm, medium format, or large format negatives, this tool adapts to your process and complements the techniques taught in Unlock the Darkroom
It's a practical extension of a deeper system for mastering your analog workflow, giving you confidence from contact sheet to final print.
Want a full system for making better black and white prints?
Darkroom Print Exposure Calculator
Frequently Asked Questions
Answers to common questions about exposure adjustments, film formats, and how this tool fits into your darkroom printing workflow.
What does this calculator actually do?
It calculates the correct exposure time when changing print sizes in the darkroom — so your prints look identical whether you're making an 8×10 or a 16×20. It works by applying either the standard “linear dimension squared” method or a more advanced bellows factor formula for large size changes.
Do I need to know my negative size?
Only if you want to use Precision Mode. The standard mode works with just your original and new print widths. If you enable Precision Mode, you’ll enter your negative width (in inches or centimeters), which gives you a more accurate exposure time when making big enlargements.
Can I use this calculator with centimeters instead of inches?
Yes — as long as you use the same unit throughout (inches, cm, or mm), the math still works. The formulas are based on ratios, so the units cancel out.
What if I’m using a different aspect ratio (e.g. cropping)?sum?
Try to measure and enter the same dimension (either width or height) from your original and new prints. If you change both width and height significantly, your aspect ratio changes — and that may affect accuracy slightly.
Does this work for all film formats?
Yes. The calculator includes a reference table with common negative widths for 35mm, medium format (120), and large format films like 4×5 and 8×10.
Will this work with fiber-based paper and variable contrast filters?
Absolutely. As long as your aperture stays the same, this calculator will give you consistent exposures — no matter what kind of paper or contrast filter you’re using.
Why is my print still off, even when using this tool?
A few things can still affect exposure: if you changed your aperture without realizing it, if the lens-to-paper distance isn’t accurate, or if your enlarger’s light falloff isn’t perfectly even. This tool gets you very close, but a quick test strip never hurts.
Who made this tool?
It was created by Matthew Koller for the Unlock the Darkroom course and darkroom printing community — to help photographers spend less time test-strip-guessing and more time making beautiful black and white prints.