人们真正会问的问题。

校准良好的屏幕测量工具，在量现成戒指时可达约 ±0.4 个美国码，在量裸手指时约为 ±0.65。限制不在数学，而在物理。在常见的屏幕像素密度下，一个像素的偏差就已经约等于五分之一码，而略微倾斜放置的卡片会量得偏短。我们会随每个结果一并给出误差范围，而不是只丢一个光秃秃的数字。

浏览器根本无从知道你的屏幕物理上有多大。同一张图在手机、笔记本和 4K 显示器上量出的毫米数各不相同。任何不先请你校准就直接画圆的工具都是在猜——而比例错误的打印对照表，比根本没有对照表更糟。

本国货币的硬币很好用：从列表中选出你的那一枚，我们会采用其官方直径。A4 或 Letter 纸同样可行，在手机上对折两次即可。你也可以用直尺，或者让我们根据你的设备型号先填入比例再由你确认。有卡并非人人如此，而默认你有卡的工具，算不上真正面向全球。

会，而且在别处这是尺码出错的常见原因。缩放改变了 CSS 像素的物理尺寸，因此缩放之前做的校准不再成立。我们的工具会察觉这一变化并请你重新校准，而不是悄悄给你一个错误数字。

它们量的东西不同。美国码是某个标尺上的位置，每一级内直径正好增加 0.8128 毫米。英国码是另一套标尺上的字母，每一级周长增加 1.25 毫米。ISO/欧盟码则直接就是以毫米计的内周长。由于级差不同，一个体系的整码很少落在另一体系的整码上——美国 6 号对应英国 L½，而不是某个整齐的字母。

借来一枚对方已经戴在正确手指上的戒指，量其内直径——这是最可靠的办法。若做不到，可以把对方的戒指压进肥皂或黏土中留下内缘印痕，或者在纸上描出内圈。若这也不行，就把对方的手指与你的比一比，然后量你自己的。成年人的平均尺码，女性约为美国 6–6.5 号，男性约为 9–10 号，但这只能当作退路，而不是计划。

通常需要。在内直径相同的情况下，宽戒圈接触手指更多，戴起来更紧。大约 4 毫米以下几乎没有影响。6 毫米时约加四分之一码，8–9 毫米时约加半码。只要你设定了戒圈宽度，我们的工具就会替你计入——而对舒适内弧的戒圈，作用方向相反，因为它们戴起来更松。

几乎在所有情况下都该往大选。稍松的戒指可以改小、加内衬或加调整珠。而过不了指节的戒指根本没法戴，硬往下褪还有卡住手指的风险。例外是特别宽或特别重的戒圈，松了会打转和下滑——这时选偏紧的那一档。

傍晚前后，在正常室温下，手既不冰凉也不浮肿的时候。手指在刚睡醒时和受冻之后最细，在受热、摄入盐分、运动或长途飞行之后最粗。在接近你日常平均状态时测量，得到的戒指能在清醒时段的大部分时间里都合适。

通常会。惯用手一般略大一些，常常相差约半码。请测量戒指最终要佩戴的那只手上的那根手指——同时也要记得，同一只手的各根手指之间同样存在差异。

全部十一种主要标准：美国与加拿大、英国及 Wheatsheaf 体系国家、采用 ISO 8653 的欧洲、法国、德国与荷兰、俄罗斯、意大利连同西班牙与瑞士、巴西、日本、中国和印度。每一种标准都由该国公开的定义计算得出，而不是从对照表里查出来的，因此各项换算彼此吻合。

不会。全部计算都在你的浏览器中完成，校准结果只保存在你自己设备的本地存储中，好让你不必重做。没有账户，没有上传，我们这边也没有任何可供删除的东西。

只有在你先核对比例的前提下才可信。许多打印窗口默认开启「适合页面」，会把整页缩小几个百分点——足以让纸上每个尺码偏移一个码以上。我们的可打印尺寸卡上有一个 50 毫米方块，你要在动剪刀之前先量一量。

选能让戒指越过指节的那个尺码，然后设法让它在指根处不打转——调整珠、戒指卡扣或稍重一些的戒圈都管用。珠宝店几分钟就能装好调整珠。若按指根尺寸选码，结果就是一枚根本戴不上去的戒指，那才是更糟的问题。

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.