Math & Units
12 free toolsUnit conversion, percentages, electrical sizing, and battery math.
Math and unit tools handle conversions and the small engineering calculations that usually mean a search for a formula: percentages and number bases, unit and byte conversions, Ohm's law and resistor color codes, LED resistors, voltage drop and wire gauge, battery and UPS runtime, and screen pixel density. Each calculator states the formula and the standard it takes its constants from.
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All 12 tools
Math Expression Evaluator
Evaluate math expressions with functions and constants
Percentage Calculator
Calculate percentages, ratios, and percent change
Unit Converter
Convert length, mass, temperature, and speed
Resistor Color Code Calculator
Decode 4/5/6-band resistor colors to ohms, or values back to colors
Ohm's Law Calculator
Solve voltage, current, resistance, and power from any two knowns
Screen PPI Calculator
Calculate pixel density from resolution and diagonal size
LED Resistor Calculator
Find the series resistor for any LED from supply voltage, forward voltage, and current
Voltage Drop Calculator
Calculate voltage drop across copper or aluminum wire runs
Wire Size Calculator
Find the AWG wire gauge for your current, distance, and voltage drop limit
Battery Runtime Calculator
Estimate how long a battery lasts from capacity, load, and efficiency
UPS Runtime Calculator
Estimate UPS backup time from battery specs and load
Bitwise Calculator
AND, OR, XOR, NOT, and shifts with a clickable bit grid at 8, 16, 32, or 64 bits
Guides
From the blog: the mechanisms, standards, and edge cases behind these tools.
- Why 12 V LED Strips Dim at the Far End: Voltage Drop Explained Current in a 12 V LED run travels out and back, so the far end gets less voltage. The round-trip formula, worked numbers, and when thicker wire or 24 V fixes it. Sep 29, 2026 · uses Resistor Color Code Calculator, Ohm's Law Calculator, LED Resistor Calculator +4
- Percentage Points vs Percent Change (Same Number, Twice) Percentage points vs percent change: a rate going 10% to 12% is up 2 points and up 20 percent at once. The formulas, the asymmetry trap, reverse percentages. Sep 2, 2026 · uses Math Expression Evaluator, Percentage Calculator, Unit Converter
- Baker's Percentages: Hydration, Starters, Yeast and Cake Pans Baker's percentages measure everything against flour, not dough. How that settles hydration, starter math, pizza styles, yeast swaps and cake pan scaling. Sep 30, 2026 · uses Percentage Calculator, Unit Converter
- How Long Does a Propane Tank Last? The BTU Math, Worked Out A 20 lb tank holds about 4.7 gallons, or roughly 431,600 BTU. Divide by the burner's BTU/h to get hours. Worked examples for grills, heaters, generators and furnaces. Sep 29, 2026 · uses Battery Runtime Calculator, UPS Runtime Calculator
- Stacked Discounts: Why 20% Off + 10% Off Is 28%, and How to Back Out Sales Tax Discounts multiply, they don't add: 20% then 10% is 28% off. To find the price before tax, divide by 1.08, don't take 8% of the total. Worked examples. Oct 14, 2026 · uses Percentage Calculator
- How to Calculate Board Feet (and Why a 2x4 Isn't 3.5 of Them) Board feet = thickness x width x length / 144. Softwood is tallied on nominal size, so a 2x4x8 counts as 5.33 BF but holds 3.5. Worked table inside. Oct 13, 2026 · uses Unit Converter
- Margin vs Markup: Why a 50% Markup Is Only a 33% Margin Margin divides profit by price, markup divides it by cost. The conversion table, how to price from a target margin, and why discounts eat margin faster than you think. Oct 13, 2026 · uses Percentage Calculator
- Bit Masks and Flags: The Bitwise Tricks Every Developer Should Know Set, clear, toggle and test bits, x & (x - 1), x & -x, packing RGB565 and Unix mode bits, and the JavaScript 32-bit traps that break masks above bit 30. Oct 12, 2026 · uses Bitwise Calculator
- Coffee to Water Ratio by Brew Method (and Why It's by Weight) The SCA's 55 g per kg of water is about 1:18. Here are the ratio ranges for pour-over, French press, AeroPress, cold brew and espresso, and why grams beat scoops. Sep 30, 2026 · uses Unit Converter
- What Do I Need on My Final? The Formula, Plus Weighted GPA Math The final exam formula is (target − current × (1 − w)) ÷ w. Worked examples, why weights must add to 100, how dropped scores shift it, and how GPA points work. Sep 30, 2026 · uses Percentage Calculator
- A CSS px Is Not a Device Pixel: PPI, devicePixelRatio, and rem The CSS px is a viewing-angle unit, not a dot on your panel. How PPI, devicePixelRatio and min-resolution connect them, and why rem sits on top of all three. Sep 29, 2026 · uses Screen PPI Calculator
- Why Your First Mortgage Payment Is Mostly Interest On a $375,000 loan at 7%, payment one is $2,187.50 interest and $307.38 principal. The two-line formula behind it, the crossover month, extra payments, and PMI. Sep 29, 2026 · uses Percentage Calculator