Free calculators for building, growing and fixing real things.
Every App Works calculator starts from a published formula or a standards-based method, then gets written up so you can see the inputs, the arithmetic and the assumptions sitting behind the number. No signup, no paywall, no upgrade tier.
Last reviewed 23 September 2026|The formula and its source are named on every calculator page
Browse by what you are trying to work out
17 categories, each one a short list of single-purpose calculators. Nothing here needs an account or a credit card.
Calculators people actually come back for
A cross-section of the library, picked for how often they get used rather than how they look in a list. Every one of them shows its formula and a worked example.
Rebar Calculator
Bar spacing, count and total length for slabs, footings and walls.
Home & RenovationPaver Sand Calculator
Base and joint sand tonnage for a paver patio or walkway.
AgricultureRaised Bed Calculator
Soil volume and bag counts for raised beds of any footprint.
Business & TradeDimensional Weight Calculator
Billable weight for parcel shipping, so you stop paying for air.
Business & TradeCustomer Lifetime Value Calculator
What one customer is worth over time, from margin and repeat rate.
Science & EngineeringOhm’s Law Calculator
Voltage, current, resistance and power from any two known values.
Science & EngineeringMotor Sizing Calculator
Torque and power a real load needs, not a nameplate rating.
PhotographyExposure Triangle Calculator
Aperture, shutter and ISO combinations for the light you actually have.
DIY & Crafting3D Print Filament Calculator
Grams and spool cost for a print, before you slice it.
TechnologyRAM Usage Calculator
Memory budget per service so capacity planning stops being a guess.
TravelTime Zone Difference Calculator
Overlap windows between two cities for calls and deadlines.
GamingCard Draw Probability Calculator
Odds of drawing what you need, for deck builders and card games.
Guides with the working shown
Long-form walkthroughs of the same methods the calculators use — the formula, a worked example and the cases where the method stops being reliable.
How a calculator gets built and checked here
Four steps, applied to every tool in the library. It is the reason some of these pages are longer than a bare input form.
Pick the method, then name it
Each calculator starts from a published formula, a recognised standard or a method in common professional use. The page tells you which one, so you are never guessing what the number is based on.
Work an example longhand first
Before a calculator goes live, the same inputs are run by hand. If the printed arithmetic and the tool disagree, the tool is wrong and it does not ship.
State the assumptions and the limits
Rounding rules, jurisdiction-specific rates, material tolerances and the cases where the method breaks down are written on the page rather than buried in the code.
Keep it correctable
Every page carries a review date. When a standard or a rate it depends on changes, the page is updated. When a reader reports an error, it is checked against the named method and fixed.
Questions worth asking before you trust a number
Do I need an account to use the calculators?
No. Every calculator runs in your browser with no account, no email and no paywall. Nothing is stored for you — reload the page and your inputs are gone.
Are the results accurate enough to build from?
Treat every result as a starting estimate. The arithmetic is checked and the method is named, but the output depends entirely on the numbers you enter and on assumptions such as material tolerances, local rates and rounding. For anything structural, regulated or safety-related, have a qualified professional confirm it before you commit.
Where do the formulas come from?
From published standards, textbook methods and manufacturer data, and the calculator page names the one it uses. Where a widely used rule of thumb is involved, the page says so instead of dressing it up as precision.
How often are the calculators reviewed?
Each page carries a last-reviewed date. A calculator is revisited when a standard, rate or threshold it depends on changes, or when a reader reports an error in it.
Do the advertisements affect the numbers?
No. Ads are served by third-party networks and are kept entirely separate from the calculation logic. The full disclosure is on our Disclaimer and Privacy Policy pages.
What if a calculator returns a result that looks wrong?
Email [email protected] with the page and the numbers you entered. The result is re-checked against the named method, corrected if it is wrong, and the page’s review date is updated when it changes.
Start with the calculation you came for
391 calculators, 17 categories, no account. If the number you need is not here yet, write in and say so.