Unit conversion tools
Bra Size Calculator
Free bra size calculator using the traditional US band-and-cup method — enter your underbust and bust measurements to get an estimated size, with the formula clearly shown.
BTU Calculator
Free BTU calculator for sizing a room air conditioner — based on the Energy Star sizing guide's square-footage table, adjusted for sun exposure, occupants, and kitchens.
Carbon Footprint Calculator
A quick, honest estimate of your annual carbon footprint from driving, electricity, and flights — using published emission factors, not a black box.
Concrete Calculator
Free concrete calculator for slabs, footings, walls, and round post holes — get the cubic yards needed and, unlike most calculators, exactly how many 40/60/80 lb premix bags to buy.
Density Calculator
Free density calculator that solves for density, mass, or volume from the other two — using the same ρ = m/V equation from either direction, so there's no separate "mass calculator" to hunt for.
Dew Point Calculator
Free dew point calculator using the Magnus formula — enter temperature and relative humidity to see the dew point and a plain-language comfort level (dry to oppressive).
Electricity Cost Calculator
Free electricity cost calculator — enter a device's wattage, how long you run it, and your per-kWh rate to see exactly what it costs to run, per day and in total.
Gas Mileage Calculator
Free gas mileage (MPG) calculator — enter distance driven and fuel used to see your real-world fuel economy and cost per mile.
GDP Calculator
Free GDP calculator using the expenditure approach (GDP = C + I + G + net exports) — plus optional growth rate and per-capita GDP if you provide a previous period and population.
Golf Handicap Calculator
Free golf handicap calculator using the World Handicap System (WHS) formula — enter your recent rounds' scores, course ratings, and slope ratings to estimate your Handicap Index.
GPA Calculator
Free GPA calculator on the standard US 4.0 scale — add each course's grade and credits, toggle weighted GPA for Honors/AP/IB courses, and optionally combine with a previous GPA.
Grade Calculator
Free weighted grade calculator — add each category (homework, tests, participation...) with its weight and your score in it, and see your overall grade instantly.
Gravel Calculator
Free gravel calculator — enter your area's size and desired depth and get the weight in tons, plus cubic yards and feet, using a standard gravel density.
Heat Index Calculator
Free heat index calculator using the NWS Rothfusz regression — enter temperature and relative humidity to see the "feels like" temperature accounting for humidity.
Height Converter
Free height converter between centimeters and feet/inches — type in either one and the other updates instantly, both directions.
Horsepower Calculator
Free horsepower calculator estimating rear-wheel horsepower from a quarter-mile trap speed or elapsed time (ET) and vehicle weight, using the standard drag-racing formulas.
Molarity Calculator
Free molarity calculator that solves for molarity, moles of solute, or solution volume from the other two — using M = mol/L from any direction.
Molecular Weight Calculator
Free molecular weight calculator — type any chemical formula (with parentheses for grouped subscripts) and get the exact molar mass using IUPAC standard atomic weights, plus the element breakdown.
Mulch Calculator
Free mulch calculator — enter your garden bed's size and desired depth and get the cubic yards, cubic feet, and number of bags needed.
Ohm's Law Calculator
Free Ohm's law calculator (the "power wheel") — fill in any two of voltage, current, resistance, and power, and the other two are calculated automatically.
Resistor Color Code Calculator
Free resistor color code calculator for both 4-band and 5-band resistors — pick each band's color from a dropdown and see the resistance value and tolerance instantly, with a visual preview of the resistor.
Roman Numeral Converter
Free Roman numeral converter — type a number or a Roman numeral into either field and the other updates instantly, both directions, with standard-form validation.
Roofing Calculator
Free roofing calculator that converts your building's footprint and roof pitch into actual roof surface area, squares, and shingle bundles needed — accounting for the extra surface a steeper pitch adds.
Shoe Size Conversion Calculator
Free shoe size converter between US Men's, US Women's, UK, EU, and foot length in cm — enter a size in any system and see it converted to all the others.
Speed Calculator
Free speed calculator that solves for speed, distance, or time from the other two — using speed = distance ÷ time from any direction.
Square Footage Calculator
Free square footage calculator for rectangles, circles, and triangles — get the area in square feet, square yards, and acres, plus a total cost estimate if you know the price per square foot.
Stair Calculator
Free stair calculator — enter the total floor-to-floor rise and get the number of steps, riser height, total run, and stair angle, with a check against typical US residential building code.
Tile Calculator
Free tile calculator that accounts for grout spacing — enter your room and tile dimensions and get the exact tile count (and box count, if you know tiles-per-box), not just room area divided by tile area.
Tire Size Calculator
Free tire size calculator — compare an original and replacement tire size to see the diameter difference and how much it will throw off your speedometer.
Unit Conversion Calculator
Free general-purpose unit converter covering length, weight/mass, volume, area, speed, and temperature — pick a category, pick from/to units, and get the result instantly.
Voltage Drop Calculator
Free voltage drop calculator for a wire run — enter the gauge, length, current, and system type, and see the voltage drop, percent drop, and voltage at the load, with a warning if it exceeds the commonly recommended 5% maximum.
Wind Chill Calculator
Free wind chill calculator using the official US National Weather Service / Environment Canada formula — enter temperature and wind speed to see the "feels like" temperature.
From the blog
A height converter with no "convert" button — type into either field
cm to feet or feet to cm — most converters make you pick a direction first. This one just watches whichever field you're typing in.
2026-09-02 Why temperature can't use the same math as every other unit converter
Multiply feet by a number and you get meters. Multiply Celsius by a number and you get nonsense — temperature scales don't share a zero point.
2026-09-02 There are two ways to calculate GDP. This calculator only does one of them, on purpose
The expenditure approach has one universally agreed formula. The income approach's exact components shift depending on which textbook you open.
2026-09-02 "Mass calculator" and "density calculator" are the same tool wearing two different names
Density = mass ÷ volume. Mass = density × volume. Same equation, same three variables, so why does every site split them into two separate pages?
2026-09-02 Speed = distance ÷ time works in all three directions — most calculators only give you one
Know your speed and time but not the distance? Rearranging the formula by hand is easy to get backwards under pressure. This just asks what you're solving for.
2026-09-02 Al2(SO4)3 has three levels of nesting most molecular weight calculators can't parse
Type a formula with parentheses into the wrong tool and it either ignores the subscript outside or just breaks. This one actually parses chemical notation.
2026-09-02 Molarity, moles, or volume — this calculator works backward from whichever two you know
M = mol/L only helps if you already know two of the three. Rearranging it under lab-report pressure is exactly when mistakes happen.
2026-09-02 IIII vs IV: most Roman numeral converters will accept the wrong one without telling you
IIII isn't standard Roman numeral form for 4 — but plenty of converters cheerfully accept it anyway, because they never check.
2026-09-02 Wire size charts skip AWG values — this voltage drop calculator computes circular mils from the actual formula
Paper wire gauge charts only list common sizes. This calculator derives circular mils geometrically for any AWG, so nothing falls through the gaps.
2026-09-02 4-band and 5-band resistors read differently — mixing them up gives you the wrong value
A 4-band resistor's third band is a multiplier. A 5-band resistor's third band is a digit. Read one as the other and you're off by a factor of ten or more.
2026-09-02 V=IR only gets you halfway — this Ohm's law calculator also covers the power equation
Know current and power but not voltage or resistance? V=IR alone can't solve that — you need the power equation too. This fills in any two of four values, not just three.
2026-09-02 Your utility bill shows the total. This calculator shows what one device actually costs to run.
A monthly bill tells you the sum of everything running in your home — it can't isolate what a single space heater or old fridge is actually costing you.
2026-09-02 A simple 4.0-scale GPA calculator gets weighted courses wrong — this one asks about Honors and AP/IB explicitly
Straight 4.0-scale calculators can't tell an AP class from a regular one, which is exactly the accuracy problem colleges warn about with generic GPA tools.
2026-09-02 Your final might be worth 30% of your grade, not 33% — a flat-average calculator can't tell the difference
Every professor weights categories differently. A calculator that just averages your scores ignores that entirely — this one lets you set each category's weight yourself.
2026-09-02 Two bra size calculators, two different answers — because they don't measure the same way
Some sites measure the band above the bust, some below, and the underlying formulas differ too. This one states its exact method instead of pretending there's one universal answer.
2026-09-02 A US 10 in one brand isn't a US 10 fit in another — shoe size conversion is only ever an approximation
Every shoe brand builds on its own last, so no conversion chart can give you a guaranteed fit across brands. This one converts between systems and says so upfront.
2026-09-02 This golf handicap calculator uses the real WHS formula — and tells you plainly it's not your official index
The World Handicap System's Score Differential formula is public. Turning fewer than 20 rounds into an accurate estimate needs the official reduced-count table too — most casual calculators skip it.
2026-09-02 Wind chill isn't defined above 50°F — a calculator that shows a number anyway is making it up
The NWS wind chill formula only applies at 50°F and below with wind above 3 mph. Outside that range, "feels like" wind chill isn't a real concept — this calculator says so instead of guessing.
2026-09-02 The heat index formula is only officially accurate above 80°F — this calculator won't pretend otherwise below that
NWS's heat index regression is validated for hot, humid conditions specifically. Below 80°F, a calculator using the full formula is showing you an extrapolation, not a real reading.
2026-09-02 50% humidity can feel bone-dry or swampy depending on temperature — dew point doesn't have that problem
Relative humidity is relative to temperature, which is exactly why the same percentage feels different on different days. Dew point gives one number that maps directly to mugginess.
2026-09-02 Your real MPG and your car's window-sticker MPG are measuring two different things
EPA ratings come from standardized lab tests. Your actual mileage depends on how and where you actually drive — this calculator measures the number that matters to your wallet.
2026-09-02 Reading trap speed off your speedometer instead of a timing slip throws off a horsepower calculation before it even starts
Speedometers are known to be inaccurate at higher speeds. Feed that into a horsepower formula and the error compounds — use your actual timing slip instead.
2026-09-02 Stay within 2% of your original tire diameter and you probably won't notice the difference — this calculator shows exactly where that line is
Bigger tires aren't automatically a problem, but past a certain size difference your speedometer, odometer, and gearing all start drifting from reality.
2026-09-02 Why two carbon footprint calculators can give you completely different numbers for the same year
Reviews of carbon calculators consistently point to the same issue: different tools quietly assume different emission factors, especially for electricity. Ours shows you exactly which numbers it used.
2026-09-02 A concrete calculator that tells you cubic yards isn't finished — you still need bag count
Cubic yards is what a ready-mix truck driver needs. Bag count is what everyone standing in the Quikrete aisle actually needs.
2026-09-01 Buying the biggest air conditioner you can find is the wrong move — here's the actual sizing math
An oversized unit cools the room fast, shuts off, and leaves the humidity behind. The right size runs longer and actually dries the air.
2026-09-01 Calculating the square footage of a circular room isn't the same formula as a rectangle
Length times width only works for rectangles. Round rooms, triangular sections, and odd angles all need their own formula — and this tool does all three plus the cost math.
2026-09-01 Uneven stairs trip people. Most stair calculators never mention why yours might be
If a staircase's riser heights aren't identical to a fraction of an inch, feet start catching on the odd one out. Here's the math that keeps every step the same.
2026-09-01 Your roof is bigger than your house's footprint — the steeper the pitch, the bigger the gap
A flat 1,500 sq ft footprint under a 6:12 roof is actually covering about 1,677 sq ft of real shingle surface. Skip that math and you'll come up short at the register.
2026-09-01 Room area ÷ tile area gives you the wrong tile count — the grout line is the missing variable
A 12-inch tile with 1/8-inch grout doesn't claim 12x12 inches of floor. It claims 12.125x12.125 — and that difference adds up over a whole room.
2026-09-01 How many bags of mulch do you need? Past a certain point, stop buying bags
A small bed is a quick bag-store run. A big one is cheaper by the truckload — the crossover point is around 2-3 cubic yards.
2026-09-01 Gravel suppliers price by the ton. Most gravel calculators still show you cubic yards
The truck that delivers your order gets weighed, not measured by volume — so "tons" is the number you actually need to place the order.
2026-09-01