Rule of Three is a focused macOS calculator for quick proportional calculations.
Enter three known values — A, B and C — and the app instantly solves for X using the classic rule of three formula:
X = B × C / A
It is designed for everyday proportion work: resizing images, scaling designs, converting measurements, comparing ratios, adjusting recipes, checking percentages, planning layouts, and solving quick math without opening a spreadsheet.
The interface is simple and distraction-free. Large input fields make each value easy to read, while the result is shown clearly and can be copied instantly. Built-in swap controls help you rearrange values faster when you need to compare different proportions.
Features:
• Solve direct rule of three calculations instantly
• Clean A : B = C : X layout
• Copy the result with one click
• Swap values quickly
• Clear all values fast
• Choose decimal precision
• System, dark and light appearance modes
• Native macOS interface
Rule of Three is made for people who need a small, fast and reliable proportional calculator always at hand.