実際によく聞かれること。

きちんと基準を合わせた画面計測なら、手持ちの指輪を測る場合でおよそ±0.4アメリカサイズ、素の指を測る場合でおよそ±0.65です。限界は計算ではなく物理にあります。一般的な画面の画素密度では1ピクセルのずれがすでにおよそ5分の1サイズにあたり、わずかに傾いたカードは短めに測れます。当ツールは数字だけを示すのではなく、必ず誤差幅を添えます。

ブラウザーには、あなたの画面が物理的にどれだけ大きいかを知る手段がありません。同じ画像でも、スマートフォン、ノートパソコン、4Kモニターでは実寸が異なります。基準合わせを求めずにいきなり円を描くツールは推測しているだけです。誤った縮尺で印刷された表は、表がないより悪いのです。

お使いの通貨の硬貨がよく使えます。一覧から選んでいただければ、公表されている直径を用います。A4やレターサイズの紙も使え、スマートフォンでは2回折ってください。定規を使うこともできますし、端末の機種から縮尺を推定して入力し、あなたに確認してもらう方法もあります。カードを持っていることは当然ではなく、それを前提にするツールは真に世界向けとは言えません。

狂わせます。他のツールでもサイズを外す原因としてよくあるものです。拡大縮小はCSSピクセルの物理的な大きさを変えるため、それ以前の基準合わせは無効になります。当ツールはこの変化を検知し、誤った数字を黙って返す代わりに、基準合わせのやり直しを促します。

測っている対象が違います。アメリカのサイズは、1目盛りごとに内径がちょうど0.8128ミリ増える目盛り上の位置です。イギリスのサイズは、1目盛りごとに内周が1.25ミリ増える目盛り上の文字です。ISO／EUのサイズは内周のミリメートル数そのものです。刻み幅が異なるため、ある体系の整数サイズが別の体系の整数サイズに重なることはまれで、アメリカの6号はきりのよい文字ではなくイギリスのL½になります。

その人がすでに目的の指に着けている指輪を借りて内径を測るのが、群を抜いて確実です。難しければ、指輪を石けんや粘土に押し当てて内縁の跡を取るか、内側を紙に写し取ってください。それも無理なら、相手の指と自分の指を見比べて自分の指を測ります。成人の平均はおおむね女性がアメリカ6〜6.5号、男性が9〜10号ですが、これは計画ではなく最後の手段と考えてください。

たいていは上げます。幅の広いアームは指に触れる面積が大きく、内径が同じでもきつく感じます。おおよそ4ミリ未満ならほとんど影響しません。6ミリで約4分の1サイズ、8〜9ミリで約半サイズ上げます。アームの幅を設定すれば当ツールが自動で反映します。内甲丸の場合は逆向きに働き、ゆるく感じられます。

ほぼすべての場合、大きいほうです。少しゆるい指輪は詰めたり、内側に当て金を入れたり、サイズビーズを足したりできます。関節を通らない指輪は使えませんし、無理に外そうとすれば指が締めつけられる恐れがあります。例外は非常に幅広で重いアームで、ゆるいと回ったり抜けたりします。その場合はきつめのサイズを選んでください。

夕方から夜にかけて、ふだんの室温で、手が冷えてもむくんでもいないときです。指は起きた直後や体が冷えたあとがもっとも細く、暑さ、塩分、運動、長時間のフライトのあとがもっとも太くなります。日中の平均に近い状態で測れば、起きている時間の大半で合う指輪になります。

たいてい違います。利き手のほうが少し大きく、半サイズほど差が出ることもよくあります。実際に着ける手の、その指を測ってください。同じ手でも指ごとに差があることも忘れずに。

主要な11規格すべてです。アメリカとカナダ、イギリスおよびウィートシーフ圏の各国、ISO 8653のヨーロッパ、フランス、ドイツとオランダ、ロシア、イタリアとスペイン・スイス、ブラジル、日本、中国、インド。いずれも表から引くのではなく各国の公表された定義から計算しているため、換算どうしが整合します。

いいえ。計算はすべてブラウザー内で完結し、基準合わせの設定はやり直さずに済むよう、お使いの端末のローカルストレージにのみ保存されます。アカウントもアップロードもなく、当方に削除すべきデータもありません。

先に縮尺を確認した場合にかぎります。「用紙に合わせる」は多くの印刷ダイアログで初期設定のまま有効になっており、用紙を数パーセント縮めます。紙面上のサイズを1サイズ以上ずらすには十分な量です。当サイトの印刷用ゲージには50ミリの正方形があり、切る前に必ず測っていただきます。

指輪が関節を越えられるサイズを選び、そのうえで付け根で回らないようにします。サイズビーズ、リングガード、少し重いアームのいずれも有効です。宝飾店ならビーズの取り付けは数分で済みます。付け根に合わせたサイズを選べば、そもそも着けられない指輪になり、そちらのほうが厄介です。

US and Canadian sizes step in halves as standard, and in quarters for finer work — a quarter size is about 0.2 mm of inside diameter. UK sizes step in halves too, written as a letter and a half, like L½. Most mass-market retailers stock whole and half sizes only, so a quarter-size result is best read as a signal for which of the two neighbouring half sizes to order. Bespoke makers will work to a quarter size if you ask.

For a ring you cannot return — a wedding band, anything custom — get sized on a steel mandrel at a jeweller. They slide graduated steel rings onto the finger and read the fit directly, which takes screen calibration out of the equation entirely. It is free at most shops and takes two minutes. Use this tool to know what to expect before you walk in, to sanity-check the number you are given, and for everything you are buying online where a shop visit isn’t practical.

Give the size in their own system, and add the inside diameter in millimetres if there is a notes field. Retailers’ charts are not always identical — some round differently, and some list a “size 7” that is really an ISO 54 — so a millimetre figure removes the ambiguity. Check the returns policy on resizing before you order, and if the band is 6 mm wide or more, tell them the width so they can allow for it.

Lay an existing ring on a flat surface and measure straight across the inside, edge to edge through the centre — that is the inside diameter in millimetres. Finding the true centre line by eye is the hard part, and half a millimetre of error is already more than half a size, so take three readings at different angles and use the largest. If you have only the finger and no ring, wrap a paper strip round it and measure the strip instead: a ruler cannot read a curve.

Look inside the band first. Many rings are stamped with their size next to the metal mark — a number for US sizing, a letter for UK, sometimes a bare “54” for ISO. If the stamp is worn or absent, measure the inside diameter and convert. Be careful with second-hand and inherited rings: a ring that has been resized usually keeps its original stamp, so the number inside can be a size or two away from what it now measures.

With two tools. A ring stick, or mandrel, is a tapered steel cone marked with sizes — an existing ring slides down it until it stops, and the mark at that point is the size. A set of ring gauges is a keyring of graduated steel bands, tried on the finger one at a time until one fits. The mandrel measures rings; the gauges measure fingers. Both are physical standards, which is precisely what an online sizer is calibrating itself towards.

Four steps, about two minutes. One: calibrate the screen by matching the on-screen outline to a bank card, a coin or a sheet of paper. Two: choose whether you are measuring an existing ring or a bare finger. Three: adjust the on-screen circle or band until it matches, viewed square to the screen rather than at an angle — viewing angle is the single most common source of error. Four: read the result, which comes with a confidence range and the equivalent size in every major national system.

Yes — this page is that tool. There is nothing to install and no account to create: you calibrate the screen once against a known object, measure, and the conversion runs entirely in your browser. Expect roughly ±0.4 of a US size when measuring an existing ring and ±0.65 from a bare finger, which is close enough for most purchases and worth confirming at a jeweller for a wedding band. Your calibration is saved in your own device’s storage so you do not have to repeat it.

Yes, and a phone is how most people use this. Two phone-specific notes. First, if you are calibrating with paper, fold an A4 or Letter sheet in half twice so it fits the screen, and tell the sizer you have done so. Second, check that browser zoom is at 100% and that you are not in reader mode or a forced desktop-site view — both change the physical size of a CSS pixel and invalidate the calibration. Screen protectors are fine; a heavily cracked screen is not, so use the printable sizer in that case.

A properly calibrated screen sizer gets you to roughly ±0.4 of a US size when you measure an existing ring, and around ±0.65 when you measure a bare finger. The limit isn’t the maths — it’s physical. At a typical screen density one pixel of misalignment is already about a fifth of a size, and a card held slightly off the flat reads short. We show you the range on every result rather than printing a bare number.

A browser has no way of knowing the physical size of your display. The same image is a different number of millimetres on a phone, a laptop and a 4K monitor. Any sizer that draws a circle without asking you to calibrate first is guessing — and a chart printed at the wrong scale is worse than no chart.

A coin from your own currency works well — pick yours from the list and we use its published diameter. So does a sheet of A4 or US Letter, folded in half twice if you’re on a phone. You can also use a ruler, or let us pre-fill the scale from your device model and confirm it. Card ownership isn’t universal, and a sizer that assumes it isn’t really global.

Yes, and it’s a common cause of wrong sizes elsewhere. Zooming changes the physical size of a CSS pixel, so a calibration done before you zoomed is no longer valid. Our sizer watches for the change and tells you to recalibrate instead of silently handing you a wrong number.

They measure different things. A US size is a position on a scale that steps by exactly 0.8128 mm of inside diameter. A UK size is a letter on a scale that steps by 1.25 mm of circumference. An ISO/EU size simply is the inside circumference in millimetres. Because the steps differ, whole sizes in one system rarely land on whole sizes in another — a US 6 is a British L½, not a round letter.

Borrow a ring they already wear on the right finger and measure its inside diameter — that’s the most reliable route by a wide margin. If you can’t, press one of their rings into a bar of soap or plasticine to capture the inner edge, or trace round the inside on paper. Failing that, compare their finger to your own and measure yours. Average adult sizes are US 6–6.5 for women and US 9–10 for men, but treat that as a fallback, not a plan.

Usually, yes. A wide band contacts more of the finger and wears tighter at the same internal diameter. Below about 4 mm it makes no practical difference. At 6 mm go up about a quarter size; at 8–9 mm about half. Our sizer applies this for you when you set the band width — and it works in the other direction for comfort-fit interiors, which run large.

Go up, in almost every case. A slightly loose ring can be sized down, padded, or fitted with sizing beads. A ring that won’t clear your knuckle is unwearable, and forcing it off risks a trapped finger. The exception is a very wide or very heavy band, where a loose fit will spin and slide — there, favour the snugger size.

Late afternoon or evening, at normal room temperature, when your hands are neither cold nor swollen. Fingers are smallest first thing in the morning and after cold exposure, and largest after heat, salt, exercise or a long flight. Measuring at your daily average gives a ring that fits for most of your waking hours.

Usually. The dominant hand tends to be slightly larger, often by about half a size. Measure the exact finger on the exact hand the ring will live on — and remember that fingers on the same hand differ too.

All eleven of the major national standards: the United States and Canada, the UK and the Wheatsheaf countries, ISO 8653 Europe, France, Germany and the Netherlands, Russia, Italy and Spain and Switzerland, Brazil, Japan, China, and India. Every one is computed from that country’s published definition rather than looked up in a table, so the conversions stay internally consistent.

Your measurement isn’t. The calculation runs entirely in your browser, and your calibration is stored only in your own device’s local storage so you don’t have to redo it — there’s no account and no upload, and no size, finger measurement or calibration value ever reaches us. Like most sites we do count page views, using Google Analytics, which records that a page was opened and can’t see what you measured on it. The privacy policy sets out exactly what that collects and how to switch it off.

Only if you check its scale first. “Fit to page” is on by default in many print dialogs and quietly shrinks the sheet by a few percent, which is enough to shift every size on it by more than a full size. Our printable sizer includes a 50 mm square to measure before you cut anything out.

Size so the ring clears the knuckle, then stop it spinning at the base — sizing beads, a ring guard, or a slightly heavier band all work. A jeweller can fit beads in minutes. Sizing for the base instead means a ring you can’t get on, which is the worse failure.