Most shops describe butt plugs as small, medium or large. The labels aren’t standardised, so a “medium” from one brand can be wider than a “large” from another, and none of them tell you what you’ll actually feel. The number that does is the width at the plug’s widest point, in millimetres. Here’s how plug sizes are measured, what the common sizes compare to, and how to work out where to start and where you might want to end up.
How a plug is measured
Three measurements describe a plug, and one of them matters far more than the others.
- Width at the widest point. Usually given as a diameter (straight across) in millimetres or inches. Some brands give the circumference (the distance around) instead; multiply a diameter by 3.14 to get it. This is the stretch your body has to accommodate, so it’s the number to compare.
- Insertable length. From the top of the base to the tip. It affects how deep the plug sits, but very little about how it feels to put in.
- Base width. The flare that keeps the plug outside the body. Wider is safer. Any plug used anally needs one.
On a Slo.Mo plug the widest point is the Peak, and below it is a narrower neck we call the Valley. You ease past the Peak, and the Valley is where the plug rests. The size on the base is the Peak.
The sizes, in millimetres and inches
These are the seven Slo.Mo plugs, which run from below the 5th percentile of erect girth to just above the 95th. The comparisons in the last column are rough, but they’re the kind of thing people actually picture.
| Plug | Width (Peak) | In inches | Around the Peak | Insertable length | Roughly |
|---|---|---|---|---|---|
| 1 | 27.5 mm | 1.08 in | 8.6 cm | 112 mm | About the width of a 20-cent coin, or a little wider than a thumb |
| 2 | 31 mm | 1.22 in | 9.7 cm | 116 mm | A 50-cent coin. The 5th percentile of erect girth |
| 3 | 34 mm | 1.34 in | 10.7 cm | 119 mm | Between a 50-cent coin and two fingers side by side |
| 4 | 37 mm | 1.46 in | 11.6 cm | 122 mm | Two fingers side by side. The average erect penis |
| 5 | 39.5 mm | 1.56 in | 12.4 cm | 124 mm | A table-tennis ball |
| 6 | 42 mm | 1.65 in | 13.2 cm | 127 mm | A golf ball |
| 7 | 44 mm | 1.73 in | 13.8 cm | 129 mm | A little above the 95th percentile of erect girth |
Australian coins for scale: a 20-cent piece is 28.65 mm across, a 50-cent piece 31.65 mm. A golf ball is 42.7 mm. For the penis comparisons we use the largest review of measured sizes to date (Veale et al., 2015): an average erect girth of 11.7 cm around, which is about 37 mm across; the 5th percentile at roughly 31 mm across, and the 95th at roughly 43 mm.
What a millimetre feels like
A millimetre isn’t the same everywhere. Going from 27.5 mm to 31 mm is a 13% increase; going from 42 mm to 44 mm is 5%. The larger you go, the more each millimetre is felt, because you’re already closer to the edge of what your body is comfortable with. That’s why the steps in the kit get smaller as the plugs get bigger: 3.5 mm, then 3, 3, 2.5, 2.5 and 2. The last step up is meant to feel as achievable as the first.
Shape matters as well as width. With a plug that stays wide all the way down, your muscles hold the full stretch for the whole time it’s in. With a Peak and a Valley, they relax once the widest point is past, so the plug can stay in comfortably for longer, and training on the Peak (easing it out and back in) becomes a few deliberate repetitions rather than a long hold.
Where to start
At the smallest size, whatever your experience. 27.5 mm is enough to feel a stretch and little enough to relax around, and the first session is mostly about learning that feeling. If it’s easy, you’ll be ready for plug 2 sooner than most. If it isn’t, you’ve started in the right place.
Where you might want to finish
That’s entirely yours to decide. Some people want to be comfortable with something around plug 3 or 4, which covers most partners and most toys. Some want the full range. There’s no size you’re supposed to reach. If you’re training towards a particular partner, measure them: a soft tape measure, or a strip of paper you then hold against a ruler, around the widest point when erect. Enter the result in centimetres into the goal finder on our size guide and it will tell you which plug matches. The same works for a toy you’d like to work up to.
Slo.Mo by Pace
Every size, drawn to scale
Seven solid silicone plugs, each a measured step up from the last, with increases that reduce from 13% to 5% as the sizes grow. A Peak to train on and a Valley to rest in, a number engraved in every base, and silicone independently tested by SGS. A$230, with complimentary express shipping in plain packaging.
Length and base
Slo.Mo’s insertable lengths run from 112 mm to 129 mm (4.4 to 5.1 inches), long enough for the Valley to sit comfortably inside, and the bases from 40.5 mm to 55 mm across, so each one is clearly wider than its Peak. Plugs 1 to 4 are medium-firm; plugs 5 to 7 are cast in a softer silicone, so the largest sizes stay comfortable.
Converting sizes
- Millimetres to inches: divide by 25.4. 37 mm is 1.46 inches.
- Diameter to circumference: multiply by 3.14. 37 mm across is 11.6 cm around.
- Circumference to diameter: divide by 3.14. A partner who measures 12.5 cm around is about 40 mm across, which is between plugs 5 and 6; the finder will point you to plug 6, the first plug whose Peak is at least that size.
To see all seven plugs drawn to scale, side by side, go to the size guide. For what to do once you’ve chosen one, read the beginner’s step-by-step guide, or how Slo.Mo works.
