General Calculation & Conversion Tools Guide (Percentage, Sizes, and Screen Size)
Some tools don't belong to a single financial or health domain, but you need them almost every week: working out a percentage, converting a size before ordering from abroad, or knowing the right screen size for your room. This guide brings together three independent tools, each with completely different logic, even though they're all "general calculators."
Guide steps
Start here for any general percentage operation: what X% of a number equals, the percentage change between two values, or the result of increasing/decreasing a number — a pure math tool with no specific commercial or academic context.
If you're shopping from an overseas store and the size is listed in a different system (US, UK, EU), convert your size here first before ordering, to avoid getting the wrong size.
Before buying a new TV or computer monitor, calculate the actual ideal size based on your real seating distance, instead of just going with "the biggest size your budget allows."
The difference between calculating a percentage and converting a size
The percentage calculator is a pure math tool: its inputs are just numbers, and its result is a direct calculation that doesn't depend on any external reference table — 20% of 500 is the same result in any country or context.
The size converter is fundamentally different: it's a match-up between established measurement charts (US size vs. EU size vs. foot length in centimeters), not a single math formula. That's why you might see slight differences between brands even with the same number entered, since some brands use cuts or manufacturing standards that differ from the global standard chart.
How screen dimensions relate to viewing distance and field of view
The ideal screen size isn't one fixed number that suits every room — it depends on your actual seating distance and the field of view you want. The closer you sit, the smaller a screen you need to fill the same comfortable viewing angle; the farther you sit, the bigger a screen you need so it doesn't feel small and distant.
This is based on two established industry standards: THX (recommends a wider viewing angle, closer to an immersive cinematic experience, around 40 degrees) and SMPTE (a slightly narrower angle, around 30 degrees, closer to professional studio reference standards). The calculator works out the right size under both standards so you can choose based on your preference.
When a general-purpose tool beats manual calculation
Calculating a percentage by hand is easy in simple cases, but it becomes error-prone for reverse operations (finding the original number from a known percentage, or getting the sign right — increase or decrease — when calculating percentage change between two values) — this is where the calculator's guaranteed-correct formula for each case pays off.
Size conversion and screen size are both hard to calculate by hand because they rely on lookup tables or trigonometric formulas (viewing angle) that most people don't have memorized — so using a dedicated tool here isn't a luxury, it's what keeps you from an actual costly guess on the wrong size.
Quick tips
- If a clothing size is listed in the EU or UK system at an overseas store, always convert it before ordering — the gaps between systems aren't a simple fixed offset across every clothing category.
- When calculating screen size, pick the THX standard if you want an immersive home-cinema experience, and SMPTE if you prefer a viewing angle closer to professional studio reference standards without eye strain.
- To get the correct discount or increase percentage, make sure you select the right direction (increase or decrease) in the calculator, since the result differs fundamentally between the two even with similar input numbers.
- Shoe size in centimeters (actual foot length) is a more reliable global reference than the number size alone, since some brands manufacture their numbered sizes with slight variations from the standard chart.
Frequently asked questions
What's the difference between the percentage calculator and the size converter?
The percentage calculator is a general math tool that works on any numbers with a fixed formula, while the size converter matches values across international clothing and shoe measurement charts — one is a calculation, the other is a reference match.
Does the size converter give a 100% accurate size for every brand?
The calculator uses standard international charts (US, UK, EU, and foot length in centimeters), and it's accurate for the vast majority of well-known brands. But some brands manufacture with slightly different cuts or sizing than the standard chart, so always check the store's own size chart when available, especially for clothing (rather than shoes) when buying from a new brand for the first time.
How is the ideal screen size calculated based on seating distance?
The calculator uses your actual seating distance and your preferred recommended viewing angle (based on whichever standard you choose, THX or SMPTE) to calculate the screen diagonal in inches that comfortably fills that angle without eye strain or needing to turn your head.
What's the difference between the THX and SMPTE standards?
THX recommends a wider viewing angle (around 40 degrees) for an immersive, cinema-like experience, while SMPTE recommends a slightly narrower angle (around 30 degrees), the standard used in broadcast and editing reference studios. Your choice between them depends on whether you prefer an immersive cinematic experience or comfortable viewing closer to professional standards.
Can I use the percentage calculator for a discount or tax calculation?
Yes, for quick general calculations — but if your context is specifically commercial (price before/after a discount, or 15% VAT), the dedicated discount and VAT calculators on the site are more precise and cover additional commercial details this general calculator doesn't.
Does screen size depend only on seating distance, or are there other factors?
Seating distance and viewing angle are the two core factors this tool calculates, but other factors like resolution (4K vs. 1080p) and usage type (gaming, movies, office work) can shift your personal preference within the calculated range — treat the result as a reliable starting point, not a single rigid number.