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Best Everyday Unit Converters You Didn't Know You Needed

Handy everyday converters for karat purity, carat weight, stone to kg, tire size, wine bottles, proof to ABV, and Elo rating — quick, free, no sign-up.

Reviewed by the thecalcu.com team · Last updated August 4, 2026

Tools reviewed in this article

Karat Purity ConverterDiamond Carat to Grams ConverterStone to Kilograms ConverterStone to Pounds ConverterFabric Weight (GSM) ConverterTire Size ConverterWine Bottle Size Names ConverterAlcohol Proof to ABV ConverterElo Rating Difference to Win Probability Converter

Overview

Some units don't come up every day, but when they do, getting the conversion wrong is easy. A karat rating gets confused with a carat weight, a UK weight in stone doesn't mean much without converting, and a tire size code looks like a cipher until you know what each number means. The converters below cover these specific, slightly obscure units that turn up in jewelry, fashion, fitness, automotive, wine, spirits, and competitive gaming contexts.

What to Look For in a Specialty Unit Converter

Correct handling of non-decimal or non-linear scales. Several of these units, proof-to-ABV, tire size codes, Elo ratings, don't follow a simple multiply-by-a-constant relationship, so the converter needs to apply the right formula rather than a naive linear scale.

Regional variants covered. Stone-to-kg, proof scales, and similar units sometimes carry historical or regional differences (like the old UK proof scale versus the US one), and a good converter accounts for which system applies.

Clear explanation alongside the number. Because you use these units less often, seeing what the result actually means in context, not just a bare number, makes the conversion genuinely useful rather than just correct.

Our Picks

Karat Purity Converter

The Karat Purity Converter translates a gold karat rating into an exact purity percentage or fineness figure, the number that actually determines a piece of jewelry's value relative to its weight.

Carat to Grams Converter

The Carat to Grams Converter converts gemstone carat weight into grams, using the exact 200-milligram-per-carat definition instead of confusing carat with the unrelated gold-purity karat scale.

Stone to KG and Stone to LBS Converters

The Stone to KG Converter and Stone to LBS Converter translate the UK's everyday body-weight unit into the metric or US measurements most fitness apps and medical charts outside the UK expect.

Fabric GSM Converter

The Fabric GSM Converter converts the grams-per-square-meter figure used to describe fabric and paper weight, handy for comparing material thickness across different product listings.

Tire Size Converter

The Tire Size Converter decodes a tire size string into its component measurements, width, aspect ratio, and wheel diameter, and helps compare sizes when shopping for replacement tires.

Wine Bottle Size Converter

The Wine Bottle Size Converter lists the volume and standard-bottle equivalent for every named wine bottle format, from a standard 750ml bottle up through Magnum and larger ceremonial sizes.

Proof to ABV Converter

The Proof to ABV Converter converts a spirit's proof rating into its actual alcohol-by-volume percentage, handling both the US 2x scale and the older UK proof system.

Elo Rating Converter

The Elo Rating Converter calculates expected win probability and the resulting rating change from a single game result, the same system used in chess and many competitive online games.

How We Evaluated

We checked each converter's formula against its authoritative definition (the exact 200mg carat weight, the US 2x proof-to-ABV relationship, the standard Elo expected-score formula) rather than relying on rounded rules of thumb. We also confirmed each tool explains what the converted figure means in practical terms, not just the raw number, and that every conversion runs instantly with no sign-up required.

Frequently Asked Questions

Karat measures gold purity out of 24 parts, so 24K is essentially pure gold, 18K is 75% gold, and 14K is about 58.3% gold mixed with other metals for durability. The [Karat Purity Converter](/karat-purity-converter/) translates a karat rating into an exact percentage or fineness figure, which matters when comparing jewelry priced by weight.
No, and people mix the two up constantly. Karat measures gold purity, while carat measures the weight of a gemstone, with 1 carat equal to exactly 200 milligrams. The [Carat to Grams Converter](/carat-to-grams-converter/) handles the weight conversion, which is unrelated to a gem's purity or grade.
Stone is still the everyday unit for body weight in the UK and Ireland, with 1 stone equal to 14 pounds or about 6.35 kg, so a weight quoted as "11 stone 5" needs converting before it means much to someone used to kg or lbs. The [Stone to KG Converter](/stone-to-kg-converter/) and [Stone to LBS Converter](/stone-to-lbs-converter/) handle both directions.
GSM stands for grams per square meter, and it's the standard way to measure the weight and thickness of fabric or paper regardless of the material's width or type. A higher GSM usually means a heavier, sturdier material. The [Fabric GSM Converter](/fabric-gsm-converter/) converts between GSM and other weight measures used for textiles.
Each part of that code means something specific. 205 is the tire's width in millimeters, 55 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 16 is the wheel diameter in inches. The [Tire Size Converter](/tire-size-converter/) breaks this down and helps translate between size codes when comparing tires or checking compatibility.
It's a long-standing tradition in the wine industry. A standard bottle is 750ml, a Magnum is 1.5L (2 standard bottles), and larger formats like Jeroboam, Methuselah, and Nebuchadnezzar scale up from there, each named after a historical or biblical figure. The [Wine Bottle Size Converter](/wine-bottle-size-converter/) lists out exactly how many standard bottles or glasses each named size holds.
ABV (alcohol by volume) is the actual percentage of alcohol in the drink, while proof is a separate scale that's exactly double the ABV in the US system, so 80 proof means 40% ABV. The [Proof to ABV Converter](/proof-to-abv-converter/) handles this conversion, along with the different proof scale historically used in the UK.
The rating adjustment depends on the expected outcome given both players' current ratings and a K-factor that controls how much a single result can move the number. Beating a much higher-rated opponent shifts your rating more than beating someone rated similarly to you. The [Elo Rating Converter](/elo-rating-converter/) calculates the expected win probability and resulting rating shift from a single result.
They do, in different ways. Karat gold purity marking is fairly standardized internationally, wine bottle naming is mostly a shared industry convention, but tire size codes and proof scales can carry regional quirks, like the UK's now-retired proof system, which wasn't a simple 2x multiple of ABV the way the US scale is.
It matters most for anyone tracking weight across UK and non-UK sources. A UK gym scale, doctor's chart, or fitness app reporting in stone needs converting before it's comparable to a US or metric reading, and small rounding differences between quick mental math and an exact conversion can matter for medical dosing or fitness tracking over time.

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