Why these tools exist
Unit conversion and date math are surprisingly error-prone. Converting 5 ft 10 in to centimetres trips up most people. Calculating how many days remain until a deadline — accounting for the current day — is a two-second task that somehow takes three Google searches. These tools do the arithmetic correctly, handle edge cases (leap years, daylight saving, metric vs imperial), and show the answer immediately so you can get back to whatever you were actually doing.
Which tool for which question
- Aspect Ratio Calculator — Width & height from any aspect ratio.
- Time Duration Calculator — Add and subtract HH:MM:SS durations.
- Percentage Calculator — Three modes: of, change, what percent.
- Date Calculator — Days between two dates, leap-year aware.
- Square Footage Calculator — Measure rooms, floors, and outdoor spaces.
- Temperature Converter — Celsius, Fahrenheit, and Kelvin.
- Random Number Generator — Custom ranges with cryptographic randomness.
- Age Calculator — Exact age in years, months, and days.
- Fuel Consumption Calculator — L/100km or MPG with trip cost estimate.
- EV Cost Calculator — Compare EV vs gasoline driving costs.
- GPA Calculator — Weighted GPA on 4.0 or 4.3 scale.
- Unit Converter — Length, weight, volume, area, speed & data.
- Timezone Converter — Convert times between world timezones.
Common conversion mistakes
- Mixing up fluid ounces and weight ounces. A fluid ounce measures volume; a weight ounce measures mass. They're only approximately equal for water — for anything else, use the cooking converter with ingredient-specific weights.
- Forgetting that Celsius and Fahrenheit don't scale linearly. You can't just multiply by a constant — you need to shift by 32 first (F to C: subtract 32, then divide by 1.8). The temperature converter handles this automatically.
- Counting date intervals off by one. The number of days between Jan 1 and Jan 3 is 2, not 3 — it depends whether you count the start day, the end day, or neither. Our date calculator is explicit about its counting convention.
- Confusing km/h and m/s. 100 km/h is about 27.8 m/s — not the same thing. If you're working with physics or engineering formulas, always check which unit the formula expects.
Frequently asked questions
How do I convert between units?
Select the category (length, weight, volume, etc.), enter your value, and pick the source and target units. The unit converter handles over 200 unit pairs across 12 categories.
What is aspect ratio and how do I calculate it?
Aspect ratio is the proportional relationship between width and height (e.g. 16:9 for widescreen). Divide the width by the height, then simplify the fraction to its lowest terms.
How do I calculate the number of days between two dates?
Subtract the earlier date from the later date. Our date calculator shows the exact number of days, weeks, months, and years between any two dates, accounting for leap years.