Before you measure: is the tank in the catalog?

A lid for a production tank does not need a tape measure. We keep a CNC template for 180+ models from Waterbox, Innovative Marine, Red Sea, Fluval, UNS, JBJ, Landen and the rest, each cut to that tank's glass. Pick the brand and model, add a feeding hole and cutouts, approve the proof. The rest of this guide is for tanks that are not listed, custom builds, and anyone who wants to understand what the lid is actually cut from.

The three numbers a lid is cut from

A MasterLids lid is a sheet of clear polycarbonate cut to the outside size of the glass, with a lip underneath that sits inside the glass. The outer edge of the lid lines up with the outer face of the tank, and the lip keeps it from sliding. That is why the custom builder asks for exactly three things:

Outside length, from the outer face of one end wall to the outer face of the other.
Outside width, from the outer face of the front glass to the outer face of the back glass.
Glass thickness, so the lip can be sized to drop just inside the walls.

Nothing is taken from the inside of the tank. The inside opening is calculated from the outside size and the glass thickness, which is more accurate than trying to hook a tape on the inside of a wet wall.

Measuring the outside length and width

Use a steel tape, not a cloth one. Hook the end of the tape on the outside face of the glass at the top edge, pull it straight across the top of the tank and read it where it meets the outside face of the opposite wall. Keep the tape level with the top edge; a tape that droops into the tank reads long.

Measure the length at both ends of the front-to-back axis and the width at both ends of the length. Glass tanks are square to within the thickness of a silicone bead, but it costs nothing to check.
Record to the nearest 1/16 of an inch or 1 millimeter. The builder takes either unit.
Measure the glass itself, not a trim strip, a silicone bead that stands proud of the glass, or a plastic frame. A rimless tank has none of these; if yours has a frame, see the framed-tank note below.

💡Write the numbers down as you go. The two most common custom-lid mistakes are mixing up length and width, and reading an inside measurement as an outside one. Length is the long side as you look at the front of the tank; width is front to back.

Measuring the glass thickness

Look at the top edge of one wall. A small ruler laid across it or a pair of calipers gives the thickness directly. Most rimless tanks use one of a few standard sheets, so if you read something close to a common size, it is that size: 5, 6, 8, 10, 12 or 15 millimeters, or 1/4, 3/8 and 1/2 inch on US-made tanks. The builder offers both unit systems.

If the walls are not all the same thickness, which happens on some larger tanks with a thicker bottom pane, measure the side walls; the bottom is not part of the lid.

Things that change what the lid can cover

Euro brace

A euro-braced tank has a strip of glass glued around the top inside edge. The lid is cut to the opening inside the brace and sits on a lip inside it, flush with the brace. Measure the opening, not the outside of the tank, and give us the thickness of the brace strip as the glass thickness.

All-in-one filter chamber

On an all-in-one tank the lid covers the display section and stops at the wall of the filter chamber, so the chamber covers, pump and media are untouched. All the catalog AIO lids are drawn that way. For a custom AIO lid, measure the display section only: outside of the front glass to the face of the chamber wall. If you would rather cover the chamber too, use the tank's full outside dimensions and the lid will span it.

Internal or external overflow

A reef-ready tank with an overflow box needs a lid that clears the box so the drains and return stay open. Catalog lids for Reefer, CADE, Fiji Cube EXT/INT, Neptunian Cube and Eshopps Mariner tanks already do. For an unlisted tank with an overflow, request a quote with the outside dimensions and a photo of the overflow from above; the lid is drawn by hand around it.

Light brackets, hang-on filters and cords

Anything that crosses the top edge of the glass needs a cutout in the lid. Those are measured separately, from the outside of the glass inward, and entered in the configurator after the lid size. The cutout guide covers width, depth and clearance.

Framed tanks

A tank with a plastic rim around the top is not rimless, and a lid with a lip inside the glass does not suit it: the rim's inner ledge is what a framed tank's lid rests on. We build for rimless and euro-braced tanks only. If you have a framed tank and want something better than the stock hood, ask and we will tell you honestly whether we can help.

What happens with the numbers

The builder draws the lid from your three measurements and shows it on screen before you pay. You add the feeding hole and any cutouts, then approve a digital proof that states every dimension the machine will cut to. The lid is CNC-cut from 3/8-inch clear polycarbonate and ships free to the contiguous US, usually within a week of your approval.

⚠️A custom lid is cut to the numbers you enter, so the fit guarantee on custom builds is a guarantee that the lid matches the proof. Check the proof against your tape before you approve it; a remake for a measurement error of your own is offered at a discount, not free. If anything on the proof looks off, reply to the proof email and we will look at it with you before anything is cut.

How to Measure a Rimless Aquarium for a Lid