Film Developer Dilution Calculator｜Split 1+9 Into Exact mL of Stock and Water
"1+9 for 500 mL — how much stock do I pour?" Stop doing dilution arithmetic on the darkroom bench. Enter a ratio and your total volume, and this tool splits it into exact millilitres of concentrate and water for one tank or tray.
💡 About this tool
One-shot dilution is the bread and butter of home development: developers like D-76, Ilfosol 3, and Rodinal are meant to be cut with water and discarded after a single use, which gives more consistent grain and saves chemistry compared with replenished stock. The catch is the mental math, and it always lands when your hands are wet and the room is dark.
"1+9" means 1 part stock to 9 parts water — 10 total parts, so one tenth is concentrate. For a 500 mL batch that's 50 mL stock and 450 mL water; for a 300 mL Paterson tank loaded with one 35mm reel it's 30 mL stock and 270 mL water. Every change of film format or tank size forces a fresh division, and that is exactly where measuring errors creep in.
Type the total and the ratio, and you get stock and water as two separate numbers you can read straight onto a graduated cylinder. Non-integer ratios work too (1+15, 3+25) — the split stays proportional.
🧐 Frequently Asked Questions
Is "1+9" the same as "1:9"? Not always, and that ambiguity is why photographers prefer the plus sign. "1+9" unambiguously means 1 part stock plus 9 parts water (10 total). "1:9" can be read either as "dilute 1 in 9 parts water" (same as 1+9) or as "1 part out of 9 total," depending on the writer. This tool treats your inputs as stock parts and water parts, so if a label says "1:9" and means 1 stock to 9 water, enter 1 and 9.
Can stock parts be more than 1? Yes. Ratios like 1+1 (equal stock and water) or 2+3 are valid; the tool partitions the total by whatever ratio you enter.
What if I set water parts to 0? You get a stock (full-strength) solution — the whole volume is concentrate.
Does this set my development temperature? No. Most developers are timed at 20°C / 68°F, but that is a separate variable from volume. This tool handles only the mix ratio; follow the manufacturer's time-and-temperature chart.
Does it work for fixer or stop bath too? Yes. Any product diluted by ratio follows the same math — enter the stock+water ratio and your total volume to get fixer or stop-bath amounts the same way.
📚 Why the "+" Notation Won
The plus sign beat the colon in photo chemistry for one reason: clarity under pressure. A colon ratio can be misread as a fraction of the whole, but "1+9" spells out the parts you add together. It is a small convention with real consequences — a "1:9" misread as "1 part in 9" instead of "1 plus 9" shifts your concentrate by about 11%, enough to nudge contrast on a delicate negative. When you borrow a recipe written in another field's shorthand, always confirm how many parts the whole is divided into before you pour.