Do I measure the outside or inside of the pan for a lid?

Measure the outside top edge, from outer rim to outer rim, because the lid rests on the rim. The inside opening is slightly smaller, especially in pans with thick walls, so measuring inside would give you a lid that is too small.

What if my square pan has slightly rounded corners?

Measure across the straight sides at the widest point, ignoring the rounded corners. Lids are made to fit the straight edges, and the corners do not affect the lid's seating on the rim.

Can I use a lid that is a little larger than my pan?

Yes, if it is no more than about 1/2 inch larger in each direction and the lid has a lip that can rest on the rim. Larger lids may not seal properly and can let heat and moisture escape.

My pan is rectangular but the lid is labeled by diameter. How do I compare?

Lids for rectangular pans are usually sold by the pan's width and length, not by diameter. If you only have a diameter, measure the diagonal of your pan and compare it, but expect a loose fit; a properly sized rectangular lid is a better choice.

How to Measure a Square or Rectangular Pan for a Lid

Why Width and Length Matter More Than Diagonal

For round pans, a single diameter defines the size. Square and rectangular pans need two measurements — width and length — because the lid must cover the entire opening, not just fit across one direction. A lid that fits diagonally will rock or leave gaps along the sides, letting heat and moisture escape.

Measuring only the longest side or the diagonal is a common mistake. The diagonal is always longer than either side, so a lid chosen by diagonal will be too large. The correct approach is to measure the two straight sides that meet at a corner, from outer edge to outer edge, and use the larger of the two as the primary size reference when comparing with lid dimensions.

Metal tape measure laid across the top of a square baking pan

Gather the Right Tools Before You Start

You only need three things: a rigid ruler or metal tape measure, a flat surface, and the pan itself. A flexible cloth tape can work but may sag, giving an inaccurate reading. A metal ruler or tape is best because it stays straight and lets you read fractions clearly.

If the pan has a rim or a rolled edge, measure across the top outer edge, not the inner base. The lid sits on the rim, so the outer rim dimension is what matters. For pans with flared sides, the top opening is slightly wider than the base; always measure at the top, never the bottom.

Metal tape measure or rigid ruler — stays straight for accurate reading
Flat work surface — so the pan sits level and the tape lies flat
Notepad and pen — record measurements before shopping or ordering

Step-by-Step: Measure the Width and Length

Place the pan on a flat surface with the opening facing up. Position the tape measure across the top, from the outer edge of one side to the outer edge of the opposite side. Keep the tape parallel to the side of the pan — do not angle it. Read the measurement to the nearest 1/8 inch (about 3 mm).

Repeat for the other side so you have both width and length. For a square pan, both measurements should be equal; if they are not, remeasure because the pan may be warped or you may have measured at an angle. For a rectangular pan, the length is the longer side, and the width is the shorter side. Write them down as width × length, for example, 9 × 13 inches.

Double-check each measurement by measuring again from the opposite side. Differences of more than 1/8 inch usually mean the pan is not square or the tape slipped. This step is worth the extra minute because lid sizes are often listed in exact dimensions, and a small error can lead to a poor fit.

Ruler placed across the top of a rectangular baking pan

How to Choose the Lid Size From Your Measurements

Once you have the width and length, look for a lid that matches both dimensions as closely as possible. Many manufacturers sell lids by the pan size, such as a 9 × 13-inch lid for a 9 × 13-inch pan. If the exact size is not available, choose a lid that is slightly larger — up to about 1/2 inch (13 mm) bigger in each direction — rather than smaller. A lid that is too small will not seat on the rim.

A slightly larger lid can still fit if it has a lip that overlaps the rim. However, if the lid is more than 1/2 inch larger, it may not create a proper seal, especially for glass lids that need to sit flush. For oven-safe lids, check the manufacturer's temperature rating to ensure it suits your cooking method.

If you are buying a replacement lid without the pan, measure the pan first and bring those numbers. Some brands list the lid size separately from the pan size, so compare your measurements with the lid's stated dimensions, not the pan's marketing name. When in doubt, contact the manufacturer with your width and length measurements — they can often confirm compatibility.

Pan measurement (inches)	Lid size to look for	Fit notes
8 × 8	8 × 8 or 8.5 × 8.5	Exact or up to 1/2 inch larger
9 × 9	9 × 9 or 9.5 × 9.5	Exact or slightly larger; check lip depth
9 × 13	9 × 13	Exact match preferred; larger may not seal
11 × 17	11 × 17 or 11.5 × 17.5	Exact or slightly larger; verify rim style

Check the Fit: What a Good Lid Should Do

A properly fitted lid should sit evenly on the rim all the way around, with no rocking or visible gaps. Gently press on one corner — the opposite corner should not lift. For rectangular pans, check both the long sides and the short ends. A slight overhang is acceptable, but the lid should not slide off when the pan is tilted.

Test the seal by holding the pan upside down with the lid on (over a sink, just in case). If the lid stays in place without tape or clips, the fit is likely good. For glass lids, make sure the rim of the pan is clean and dry, as food residue can prevent a proper seal. If the lid rattles or moves easily, it is too large and may let steam escape, affecting cooking times.