Choosing Light Bulbs: Caps, Lumens, Colour and Dimming

Close up of a clear glass globe with a filament bulb on the Clifton floor lamp

The right bulb makes a good light look its best. The wrong one can glare, flicker or simply not fit. Here is what the codes and numbers on the box mean, and how to choose.

Start with the cap

The cap is the end of the bulb that fits into the lamp holder. The letter shows the type. The number is a size in millimetres, either the width of the cap or the gap between the pins. Your light's product page or label will tell you which cap it takes and its maximum wattage. Never fit a bulb above that maximum, and always switch off and let the old bulb cool before you change it.

Cap What it is Typical uses
E27 (ES) Edison screw, 27 mm across Pendants, table lamps, floor lamps, larger ceiling lights
E14 (SES) Small Edison screw, 14 mm across Chandelier arms, candle bulbs, smaller wall lights and pendants
B22 (BC) Bayonet cap, 22 mm, push and twist Many traditional UK pendants and lamps
GU10 Mains voltage spotlight, two pins 10 mm apart, twist to lock Downlights and spotlights
G9 Small mains voltage capsule, two looped pins 9 mm apart, pushes straight in Wall lights and compact ceiling lights

Lumens, not watts

Watts measure the power a bulb uses. Lumens measure the light it gives out. LEDs make far more light per watt than old bulbs, so compare bulbs by lumens. The Energy Saving Trust gives these rough equivalents.

Old incandescent bulb LED brightness to look for
40 W About 470 lumens
60 W About 800 lumens
75 W About 1,050 lumens
100 W About 1,520 lumens

In a fitting with several bulbs, add them together. Five bulbs of 400 lumens give about 2,000 lumens in total.

Warm white or cool white?

Colour temperature describes how warm or cool white light looks. It is measured in kelvin (K). The lower the number, the warmer and more golden the light.

  • 2700K, very warm white: cosy and relaxed. Ideal for living rooms and bedrooms.
  • 3000K, warm white: a little crisper but still warm. Suits most rooms.
  • 4000K, cool white: bright and neutral. Some people prefer it in kitchens and bathrooms.

Some decorative bulbs go warmer still, around 2200K, for an amber glow. Try to keep to one colour temperature in each room so your lights match.

CRI: how true colours look

The colour rendering index (CRI, sometimes shown as Ra) measures how faithfully a light shows colours, on a scale up to 100. For most indoor lighting, CRI 80 is the recommended minimum. CRI 90 or more makes wood, fabric, food and skin tones look more natural, which is worth having in kitchens, bathrooms and dressing areas.

Dimmable LEDs and dimmer switches

Not every LED bulb can be dimmed. If your light is on a dimmer, choose bulbs clearly marked as dimmable.

The dimmer switch matters too. Older dimmers made for halogen or traditional bulbs often cause flicker or buzzing with LEDs. A modern LED dimmer, usually a trailing edge type, is a better match. Check that the total wattage of the LEDs on that switch sits within the dimmer's minimum and maximum load.

Results can still vary between brands, so check compatibility with the bulb maker where you can. Do not mix dimmable and non dimmable bulbs on the same dimmer. Fitting a dimmer switch is electrical work, so have a qualified electrician fit it. The same goes for installing any hardwired light.

Bulbs for clear glass shades

In a clear glass shade, the bulb is on show and becomes part of the design. Decorative LED filament bulbs, which recreate the look of old style filaments, are the classic choice. Globe shaped bulbs give a softer, rounder look. A warm white of 2700K or below suits most clear and smoked glass.

Check that the bulb fits inside the shade with space all round, as a large globe will not fit a small shade. If a clear bulb feels too bright to look at, a frosted or opal bulb softens it. The Thorpeness Rose Glass Pendant Light comes in clear or frosted glass, so you can choose how much of the bulb you see.

Bulbs for opal and frosted glass

Opal glass is milky white. It spreads light evenly and hides the bulb, so any standard LED bulb with the right cap and size works well. Choose the lumens for the job and a colour temperature to suit the room. The 20 cm Selwyn Opal Glass Globe Wall Light is one example. Frosted glass works in a similar way, though a frosted bulb helps to avoid a bright spot.

Browse our glass lighting to compare shades.

Built in LEDs

Some fittings, such as many LED vanity lights and bar lights, have built in LEDs instead of a bulb. There is nothing to swap later, so check the brightness and colour temperature on the product page before you buy.

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