The practical stuff, handled.
Finance calculators, privacy tools, statistics utilities, and health checks. Everyday tools that don't need a login or a subscription.
Every calculator here shows its formula.
Search for “COGS formula” on a Tuesday night and Google hands you twenty SEO-bait pages with a dirty calculator buried below the fold. Search again Thursday morning and you type the same numbers again, because nothing saved. There's a quiet tax on that workflow, multiplied by every calculation a normal person quietly Googles two or three times a month.
Where the formulas come from
The usual alternative is a spreadsheet cell with a formula you half-remember. That works the first time. Two weeks pass, you open the sheet, and you're squinting at =(B2-B3)/B2 trying to recall whether you divided gross profit by revenue or the other way around. Google it. Rebuild. Run the number. Close the tab. Forget it by next month. No single instance is expensive. The pattern just compounds quietly, and nobody notices. The other option, the first calculator on page one, is almost always owned by someone who'd like to sell you something related to the number you just typed.
So every calculator in this toolkit states its method on the page, next to the result. You get a number and you see how. When a professor or a boss asks how you got there, you can show the steps. A decent tool explains itself with the math it used, and these do it literally: Massvo substitutes your numbers into the density formula line by line, Parify prints the conversion formula directly under the converted rate, and Cogsly opens a breakdown panel that walks the arithmetic row by row.
Naming the method also forces the edge cases into the open. Four distributions for a p-value, not one. Custom ABV per drink, since a 9% IPA isn't the same as a 5% lager. Freight-in as a proper line item, not a footnote. Eighteen real materials in the density table, not one generic formula. Default forms are wrong for most real situations, and a tool that shows its work can't quietly round those situations off.
There's a second half to trust. A BAC calculator that phones home is a BAC calculator you shouldn't trust. Same with your down payment, your margin, your test statistic, your sample weight. The math isn't the sensitive part. The inputs are, which is why the math here runs in your browser and the inputs stay in it.
One margin check, start to finish
Here's what a stated method looks like in practice, using Cogsly's own fields. Say the quarter opened with $12,000 of inventory. You purchased $7,500 more, paid $600 of freight-in (a direct expense, so it gets its own line), and closed the quarter holding $9,200. Revenue for the same period was $28,000.
COGS is $10,900, gross profit is $17,100, and $17,100 over $28,000 of revenue is a 61.07% gross margin. Every intermediate value stays on screen, so when the result disagrees with what your accountant sent back, you can see exactly which line disagrees instead of re-deriving the whole thing.
Formula provenance
| Tool | Method | Provenance |
|---|---|---|
| Sober | Widmark formula | The formula used in forensic toxicology, with custom ABV per drink. |
| Accruo | Compound interest with periodic contributions | Validated within a dollar of SEC Investor.gov reference output. |
| Pvalr | Z, T, chi-square, and F distributions | Computed client-side through jStat. |
| Perclo | Monthly payment with PMI priced by LTV band | 3%, 5%, 10%, and 20% down compared side by side. |
| Parify | APR to APY conversion, both directions | Eight compounding frequencies. The exact formula prints under the result. |
| Massvo | Density, mass, and volume, solved for any variable | 24 units. An 18-material reference table prefills density. |
| Cogsly | Beginning inventory + purchases + direct expenses - ending inventory | The breakdown panel shows every line as it computes. |
Straight to the right tool
Nine tools, nine jobs. Two of them work on files instead of figures: Blank previews what's sitting in a photo's EXIF header (GPS coordinates, device serials, timestamps) before you wipe it, and Komprs hands its compression queue to a Web Worker so a long batch doesn't stall the tab, with a split before/after viewer that shows where the artifacts start. The rest take numbers. Pick by the task, not the name.
None of this is exotic math. Widmark published his formula in 1932, and the COGS identity sits in the first week of every accounting course. What the toolkit adds is narrower than that: the working stays on screen, and the form keeps the edge cases a default calculator rounds off. The next time a number matters enough to double-check, check it against a calculator that can show you where it came from.
Everyday Tools
See what your photos are hiding, then strip it.
Phones bake location, device model, and often the serial number straight into every photo they take. Social platforms sometimes strip that header before posting. Often they don't. Blank reads what's in there before you share. GPS coordinates, device IDs, timestamps, plus whatever else the camera manufacturer decided to embed. Batch processing handles up to twenty images at once, with a one-click wipe on the output and a ZIP download when you're done. Everything runs in the browser, so nothing uploads. JPEG, PNG, WebP, HEIC all supported.
Try it 02Compress unlimited images without a single upload.
TinyPNG and Optimizilla batch-compress images by uploading them to a server. Squoosh keeps files on-device but only handles one at a time. Komprs runs an unlimited batch entirely in the browser through OffscreenCanvas and a Web Worker, so the files never leave the machine. A global quality slider updates estimated output size across the whole queue in real time, and a split-slider before/after viewer moves with it so you can watch where the artifacts start. JPEG, PNG, and WebP in and out. Download per file, or grab the whole set as a zip.
Try it 03COGS, gross profit, and margin in one calculation.
Every free COGS calculator online gets you halfway. You type your numbers, get cost of goods sold back, and then open a separate tab for gross profit and margin. Cogsly does all three at once. Freight-in and direct expenses are first-class fields instead of a footnote, and the step-by-step formula breakdown shows the arithmetic as it happens. That matters when you're checking the number against what your accountant sent back. Nothing hits a server. Built for Shopify and Etsy sellers who shouldn't need accounting software for a margin check.
Try it 04Every down payment scenario, side by side.
"How much do I need for a down payment?" has at least four right answers. Each one shifts your monthly payment. Some trigger PMI. All of them change what you need in cash at closing. Most calculator sites hand you one scenario per page, so you end up with four tabs open. Perclo puts 3%, 5%, 10%, and 20% side by side with real PMI rates by LTV band and total closing cash on the same line.
Try it 05The compound interest calculator that doesn't get in your way.
Incumbent compound interest calculators make you press a button to see the result, then bury the year-by-year table under a signup funnel. Accruo skips both. Initial balance, monthly contribution, rate, and years are linked sliders and number fields. The stacked chart updates as you drag, splitting principal, contributions, and interest. The full year-by-year table sits right below, no pagination, sticky header. Five contribution frequencies, four compounding frequencies. Validated within a dollar of SEC Investor.gov reference output. Nothing leaves your browser.
Try it 06Test statistic in, p-value out. 30 seconds.
Looking up whether a test statistic of 2.31 clears significance at alpha = 0.05 with 14 degrees of freedom used to mean digging through a table in the back of a textbook, or opening R for a one-line call. Pvalr does it in a form field. Z, T, chi-square, F. One-tailed or two. Custom alpha. Degrees of freedom where they belong. Output is the p-value, a plain-English verdict on whether it clears the bar you set, and a reminder that low p-values don't measure effect size. Client-side through jStat.
Try it 07Am I intoxicated? How long until I'm sober?
Most BAC calculators assume standard drink sizes and strengths, so the number's wrong the moment you're measuring anything real. Sober takes custom ABV per drink. A 9% IPA and a 5% lager aren't the same drink, and a bar double isn't either. The math is the Widmark formula, the one used in forensic toxicology. You get a color-coded result against the legal limit plus a countdown to when your estimated BAC hits zero. Nothing saved, nothing sent.
Try it 08Density, mass, volume — solved and explained.
Density, mass, volume. You know two, you want the third. Massvo solves in any direction, mixes metric and imperial freely (pounds for mass, liters for volume, g/cm³ out if that's what your spec sheet wants), and walks through the formula with your numbers substituted so you can actually show your work. Eighteen materials sit in a reference table. Click any one to prefill the density and solve from there. A buoyancy indicator tells you whether the result would float in water without making you look it up.
Try it 09The clean APR↔APY converter — your answer in 10 seconds.
The #1 APR-to-APY result on Google is an ad-cluttered WordPress relic running ads for movie streaming and coloring pages. Parify is the opposite: a single-purpose, mobile-first converter that hands you the answer the instant you finish typing. Enter a rate, pick how often it compounds — daily for credit cards, monthly for savings, quarterly for CDs — and watch the honest equivalent appear, with the exact formula and your compounding period shown beneath it. Both directions live in one clean UI; most competitors split that across pages. The math runs entirely client-side, nothing leaves your browser, and there is no bank cross-sell, crypto signup, or mortgage pre-qual. Just the conversion you came for.
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