Mirco make one size of each shell and size the driver by weight and height rather than by waist. The trick is picking the shell built for your build.
How Mirco size their seats
Mirco publish a maximum driver weight and height for each FIA model rather than a waist. The GT is the slim one: up to 85 kg and about 170 to 185 cm. The RS2 goes to 95 kg and 190 cm, the RTS-2 to 100 kg and 190 cm, and the Classic to 100 kg and 185 cm. The RTX-7 is the widest shell in the range at 600 mm across the shoulders.
Across the whole range the shoulder width runs from 490 mm on the Flyer to 600 mm on the GTM and RTX-7. Our waist bands put nearly every Mirco at 32 to 36 inch, with the RTS, RTS 2 and XL also listed at 36 to 40. If you are at the top of a band, take the wider shell.
The RTX-7 in numbersShoulder width 600 mm, hip bolsters 490 mm, lap belt slots 385 mm apart, base 420 x 615 mm, backrest 645 mm, overall height 920 mm, side mount holes 290 mm apart, 9 to 10 kg depending on trim.
Which Mirco fits you
Mirco driver limits
Mirco’s own figures for the FIA shells. They describe them as approximate, so treat a driver right on a limit as a candidate for the next shell up.
All the FIA shells are side mount with the holes 290 mm apart, five harness slots and HANS friendly.
Measure yourself first
Four numbers decide whether a seat fits. Take them in the clothes you drive in, and write them down before you compare models.
AWaistAround your natural waist, not your trouser size. Trouser sizes usually run an inch or two under the real figure, and the waist bands on our product pages use the real one.
BSeated hip widthSit on a hard chair with your feet flat and measure straight across the widest point of your hips with a rule or a straight edge, not a tape that follows the curve. This is the number that decides whether a fixed shell is tight.
CShouldersStraight across the back at the widest point. Fixed back seats hold you here, so a shell that is narrow at the shoulders is uncomfortable long before it is unsafe.
DHeight and torsoYour height, and if you are over 6 ft whether that is mostly leg or mostly body. A long body needs a taller shell with the head support set higher.
Before you order
- Check the car, not only the driver. Transmission tunnels, door cards and B pillars are what stop a wide shell going in. Our seat mounting and fixing points guide covers what to measure.
- Side mount and base mount shells are not interchangeable. Which one you need depends on the car and the subframe, so pick the brackets at the same time as the seat.
- If you run a HANS or plan to, filter on HANS friendly. The shoulder openings are cut to clear the collar.
- FIA seats carry an expiry date on the homologation label. Check what your championship accepts and how long the seat has left, particularly on discounted older stock.
Send us your measurementsWaist, seated hip width, height and the car. We have fitted a lot of Mirco seats and will tell you honestly if the one you have picked is going to be tight. Call 0115 989 3488 or email Sales@GSMPerformance.co.uk.
Waist bands and heights are our product data, checked 6 September 2026. Driver limits, weights and the RTX-7 dimensions are from Mirco’s product pages, read the same day.
