Calculate percentages
Four common percentage problems in one place — X% of Y, X is what % of Y, X is Y% of what, and X-to-Y change.
How percentages work
Pick the mode that matches the question, type two numbers, get the answer and the formula.
- 01
Pick mode
Four common questions covered.
Inputs are treated as UTF-8 throughout the pipeline and validated client-side as you type — nothing dispatches until the value is well-formed.
modeinpututf-8client-sidevalidatedQ · text% of · setX% of Y
loaded · parsedclient-side - 02
Enter X
The first number — meaning depends on mode.
Each toggle re-computes through a memoised hook, so flipping options never resets your input and the UI stays snappy on large inputs.
xoptionslivememoisedtogglesX · memoised20 · liveX = 20
options set · memoisedlive - 03
Enter Y
The second number.
Randomness — where used — comes from
crypto.getRandomValues, neverMath.random. Heavy work sits inuseMemo.ycomputeinstantlocaldeterministicY · instant250 · computedY = 250
computed · instantinstant - 04
Read result
Answer plus formula are shown.
Copy writes through
navigator.clipboardand downloads use aBlob— zero network traffic. Same input plus same options always yield the same bytes.readcopydownloadzero-networkreproducibleok · copiedready · copied50
shipped · ready to shipsuccess
Pro tips for everyday utilities
Reach for crypto.getRandomValues
Cryptographically random for IDs, tokens, salts. Math.random is for animation and games, not security.
Mock data with seeds
Faker-style libraries can be seeded so tests are reproducible. Embed the seed in the fixture file.
Reference cards beat memory
Cheatsheets for Vim, Git, Docker, Linux signals are faster than man pages when you’re mid-flow.
Browser APIs vary
Touch events, gamepads, and clipboard differ across vendors. Feature-detect before use.
Persist in the right place
localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data.
Everyday percentages
Shopping, school, work.
Shopping discounts
20% off $250. Random values come from crypto.getRandomValues — cryptographically secure for tokens, salts, and session IDs. Math.random is deterministic enough for animations but predictable enough to be a security bug.
CSPRNG-backedGrade scaling
Score 47/60 as a percentage. Pair with hashids, ULID, or UUID v7 when you need both random and sortable — useful for primary keys, public-facing IDs, and event log ordering without a sequence column.
reproducibleTip math
Quick tip lookup at restaurants. Output is deterministic given the same seed, so fixtures and tests stay reproducible across CI runs and contributor machines. Embed the seed in the fixture file for full replay.
snappyGrowth rates
Q1 vs Q2 revenue change. Heavy work runs through useMemo so the UI stays snappy on large inputs. Same input + same options yields identical bytes — the kind of stability snapshot tests rely on.
feature-detectedAllocations
X% of budget toward Y. Storage tools persist to the right backend: localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data. Pick the right shelf and quota worries disappear.
deterministicConversion rates
Sign-ups as % of visitors. Browser feature detection: clipboard, fullscreen, gamepad, and Web Bluetooth APIs gracefully fall back when unavailable. No surprise white screens on Safari or older Firefox.
production-readyFrequently asked questions
Something missing?
Tell us what you need. We ship the top requests every two weeks.