The new bulb does not fit the shade: size and shape checks

Why a new LED bulb may not fit an old glass shade, which dimensions to measure, and how to pick a shape that will go in.

You bring home a bulb with the same cap and the same wattage, but when you try to refit the glass shade the screws will not reach, or the tip of the bulb presses against the bottom of the glass. This happens a lot when people move to LED. The reason is simple: the cap is the same, but the body of the bulb has changed.

Why an LED bulb can be larger

An incandescent bulb was just a glass envelope and a cap. An LED bulb has a driver between the cap and the dome, and a body that works as a heat sink to draw heat away from the diodes. Those parts add length. The higher the output, the larger the body needed to spread the heat, so high-wattage LED bulbs can be noticeably bigger than the old bulbs of the same shape.

The shape name is no guarantee either. "A60" on the box tells you the diameter is about 60 mm, but the length varies from maker to maker. Two bulbs with the same shape code can differ by a few centimetres.

Which dimensions to check

Three measurements have to fit inside the shade, and each can cause its own problem:

• Length. The distance from the end of the lampholder to the bottom of the shade. Remove the shade and measure the space inside, or compare the old bulb's length and width with the dimensions printed on the new packaging.
• Diameter. The widest part of the bulb should not touch the sides. A tight fit heats the shade and can crack some types of glass.
• Neck opening. Globe and pear-shaped shades have a narrow mouth. If the shade goes on over a bulb that is already screwed in, the bulb must be narrower than that opening.

Look for the dimensions on the box. Most manufacturers state diameter and length in millimetres. If they are missing, asking the shop to open a box and measure takes a minute; at home it means another trip to return it.

What to do when it does not fit

If a standard A60 will not go in, there are more compact shapes. A small G45 globe or a C37 candle bulb with the same cap can be both shorter and slimmer, and these shapes are designed for tight shades. A smaller bulb usually gives less light, though, so if this fitting is the main light in the room, check the wattage and lumens.

How the shade is fixed also makes a difference. Some shades are clamped by a threaded ring on the lampholder. The shade then sits around the holder, and the bulb must be shorter than the depth of the shade. Others are held to the fitting by side screws; there is a few millimetres of play, but tightening the shade on at an angle just to make it fit is not the answer.

Extending the bulb with an adapter does not help; it pushes the bulb even closer to the bottom of the shade.

With table and floor lamps the question is slightly different. The bulb may not touch the shade, but its tip can hang below the lower edge and shine straight into the eyes of someone sitting nearby. The metal frame that carries the shade decides how high the bulb sits. Choosing a shorter bulb, or a shape that stays inside the shade, protects both the look and your comfort. With fabric shades, keep some distance between bulb and fabric too, as a hot spot will yellow the material over time.

Heat inside an enclosed shade

Even when a bulb fits, a closed shade with no ventilation is a hot place for an LED. When the diodes and driver cannot shed heat, the bulb ages faster. It makes sense to leave an air gap between bulb and glass, to avoid choosing the highest-output model unless you need it, and to prefer bulbs marked as suitable for enclosed fittings.

Quick check

• The old bulb's length and diameter have been measured.
• The neck opening and depth of the shade are known.
• The new bulb's dimensions on the box have been compared.
• There is a gap between the bulb and the side of the shade.
• The shade sits straight and fully home, with no forcing of the screws.

Alongside standard E27 bulbs, the Zəfər İşığı range includes more compact shapes such as the C37 E14 candle and the C45 E27. Narrow shade? Sorting the catalogue by bulb shape will narrow the options fast.