A tool that works well embedded in an article passes one test: it needs nothing but itself. No sign-in, no upload, no round trip to a server, no explanation sitting above it. You drop it into the middle of a paragraph about mortgages and it answers the question the paragraph raised, without the reader going anywhere.
Almost everything here passes that test, because everything here was built to run in the browser in the first place. What follows is the catalogue by family, with the kind of article each one tends to belong in.
Converters and quick maths
The oldest and least glamorous family, and the one people actually search for: kg to lbs, cm to inches, Celsius to Fahrenheit, km to miles, litres to gallons. They belong in recipes, shipping guides, travel posts and anywhere a number arrives in the wrong unit.
Alongside them sit the small calculators that answer a question in one line — percentage, average, fraction, ratio, tip — and the scientific calculator for when the arithmetic gets longer than a sentence.
These are the ones to reach for when the tool is a footnote to your writing rather than the subject of it. They are short, they need one input, and they do not push the rest of the article down the page.
Money, in three different moods
Personal finance is the heaviest family: mortgage, loan and car loan, compound interest, retirement, inflation, paycheck. These are long tools — a mortgage calculator produces a full amortisation schedule — so they work best as the centrepiece of a post rather than an aside in it.
Business and commerce is the second mood, and the shortest: VAT and reverse VAT, margin, markup, break-even, discount stacking, marketplace and eBay fees. If you write for shop owners, these are the ones that get bookmarked.
Marketing is the third: ROAS, CPA, CPC, CPM, CTR, conversion rate, customer acquisition cost, lifetime value. An agency blog explaining break-even ROAS is a better page with the calculator in it than with a worked example in a table.
The body
Health tools carry a different obligation: someone is going to act on the number. BMI, BMR, TDEE, body fat, macros, ideal weight, one-rep max, pace, heart rate, water intake. Pregnancy and fertility have their own set — due date, ovulation, baby age.
Every one of these carries its method and its sources with it into the frame, which matters when the number is going to be repeated by someone who did not read your article carefully.
The workshop
The largest family by some distance, and the most specific. Electrical: resistor colour codes in four, five and six bands, resistors in series and parallel, voltage dividers, LED resistors, voltage drop, cable size, AWG conversion, PCB trace width. Power and energy: watts to amps, kW to amps, three-phase, battery runtime and charge time, solar panel and inverter sizing, electricity cost.
Then the building side: paint, tile, grout, wallpaper, flooring, decking, concrete, cement, gravel, sand, mulch, fence posts, stairs, roof pitch, room and wall area.
These reward a specific article rather than a general one. A post about wiring a garage is exactly where a voltage drop calculator belongs, and nowhere else is.
Dates, and the calendar’s odd corners
The age calculator is the most-embedded tool of this kind anywhere, and the rest of the family is quietly useful: days between dates, business days, date arithmetic, time zone conversion. Anything involving "how long until" or "how long since" lives here.
One thing worth knowing: every date tool resolves in the visitor’s browser, not on a server. The answer is correct for the reader’s own clock, wherever they are and whenever they arrive.
Chance
Pickers and randomisers are the family that gets used rather than read: the wheel spinner, yes-or-no wheel, random name picker, dice, coin flip, random numbers.
They are the best argument for embedding rather than linking. A classroom blog, a giveaway announcement, a team page — the reader does not want to visit a tool site, they want to spin the thing and get on with it.
Text, colour and codes
Smaller families, but each has one tool that keeps being needed. Text: word counter, morse, binary, word unscrambler. Colour: the colour picker, with conversion between hex, RGB and HSL. Codes: the QR generator and the barcode generator, both of which draw the result in the browser and hand it over as a file.
The exception: anything that ends in a file
Three families stay here rather than travelling: the PDF utilities, the image converters and the invoice maker. They are perfectly good tools, and they are the wrong shape for a frame on someone else’s page — they finish by handing you a file, and a sandboxed frame is a poor place to receive one. Link to those instead of embedding them.
If you publish
The WordPress plugin covers the whole embeddable catalogue: add a block, search for the tool, publish. Everything else here — automatic height, lazy loading, isolation from your theme, no tracking cookies — is the plumbing, and it is deliberately boring.
The rule of thumb that matters more than any of it: embed the tool that answers the question your paragraph just asked. One tool, in the right place, is worth five in a row at the bottom.