Caps, shapes and styles – choosing a bulb that fits

You just want a replacement light bulb, but with so many different bases (caps), shapes and sizes, it’s not always so simple to find one that fits.

To work out what’s best for you, take your old bulb with you to the shop. Otherwise, to learn more about the options available, explore this page to find out about the different light bulb caps on the market – and what shapes and styles you’ll come across.

What base or cap do I choose?

The lamp ‘cap’ or ‘connector’ is the base of the bulb that plugs or screws into the lamp fitting or socket.

The easiest way to work out what cap you need is to take your old bulb with you to the shop. You’ll find the most common caps are either screw (Edison screw) or bayonet, though there are a range of sizes of each.

There are also other caps used with downlights, spotlights, capsules, tubes and other bulbs. To learn more about different options, refer to the comparing common caps table below.

Table 1: Comparing common caps

Mains voltage (240V)


The most common cap in Australian and New Zealand homes is the bayonet. It secures to the socket (fitting) by a twist-lock.

The standard diameter of mains powered bayonet caps is 22mm and the reference number for this cap is B22 (Bayonet, 22mm). Another common bayonet cap is the 15mm (B15).


Compact, mains powered halogen and LED downlights usually have two pins sticking out the back of the cap (a bi-pin cap). These pins connect to the fitting via a twist lock, similar to how bayonet caps fit.

The most common has a GU10 cap.



The screw cap is also known as the ‘Edison screw’, which is standard in the US and common in Europe – though still used widely in Australia and New Zealand.

The standard size of screw fitting is the E27 (27mm diameter). However there are other common sizes used on mains fittings in Australia such as the E14 also known as the Small Edison Screw (SES) often found in candle light bulbs in chandeliers and decorative table lamps.



Mains powered circular and straight (linear) fluorescent tubes usually have bi-pin (2-pin) caps at one end – sometimes at both.

The most common cap (connector) on fluorescent tubes in Australian and New Zealand homes is the G13, which is on the tube referred to as the T8. Another common connectors is the G5, which is found on the even smaller diameter tube called the T5.


Some compact tubes have one connector at each end, rather than both pins/connectors on one end.

The most common cap for these compact tubes, in Australia and New Zealand is the R7, either regular length or compact. You’ll often find these in outdoor flood lights around the BBQ and in gardens.

Low voltage (12V)



Low voltage MR16 downlights use a GU5.3 bi-pin cap. It’s more compact than the mains voltage equivalent using the GU10 cap.

If you are replacing a halogen MR16 lamp with an energy efficient LED light, you may need to consider your home’s wiring. Low voltage lamps need a transformer to step it down to 12V. Not all transformers are compatible with all LED lamps – and some existing dimmer switches won’t work either.

The E3 Program is currently working on a resource to match compatible LEDs with dimmers and transformers. In the meantime, if you’re replacing a halogen MR16 with an LED, ask your electrician or a specialist lighting retailer for advice.



Smaller capsule light bulbs use bi-pin caps in a range of sizes. The G4 and GY6.35 capsules are commonly found in desk lamps, table lamps and other applications where they need to be compact.

As with the MR16 downlights, low voltage capsule lights also need a transformer between the wall plug and lamp fitting. Often the transformer is contained in the base of the desk or table lamp – so you can’t see it. Because there is a transformer, you may experience compatibility issues if replacing on old halogen with some new LED lights.

If in doubt – or if you experience any compatibility issues such as flickering – take the old bulb and table or desk lamp with you to the shop. A specialist lighting retailer (or an electrician) can help ensure you choose an energy efficient LED that will work best. 

Light bulb shapes and styles

We’re all familiar with the traditional, pear shaped omnidirectional (emits light in all directions) light bulb – and a lot of the time that’s all you need.

However, there are a number of other shapes and styles on the market, including retro or vintage-looking bulbs that may look old but inside they can have modern technology. You’ll also find spotlights and downlights, spiral CFLs and tubes – as well as variations of each for practical or aesthetic reasons.

To choose the right shape or style, take your old bulb with you to the shop – It’s the sure way to pick one that fits.

Otherwise read on for more information about what other options you have available – including which new, energy efficient options you could choose that will physically fit, suit your purpose, give you the look you like and save you money on electricity bills.

Traditional light bulbs


The old fashioned, omnidirectional pear-shaped glass bulb is one of the most common styles available.

However, what’s inside the bulb is quite different. Nowadays, energy efficient LED lights can be housed in a pear-shaped glass or plastic enclosure to mimic the traditional or older retro look. This means they can serve as direct replacements for incandescent lamps, because they physically fit in the same space – and use the same cap.

Vintage light bulbs


Many lighting retailers also sell decorative and retro light bulbs, shaped to look like rounded or skinny vintage bulbs.

If you are buying a vintage light bulb, always check the physical space around your light fitting first. Being a non-standard shape means they won’t fit under some lamp shades or in oyster lamps.

You’ll also want to make sure you buy a vintage light bulb that won’t give you bill shock – some old-looking bulbs will cost you a lot more to run…

This is because some vintage or retro light bulbs available in Australia use the old-fashioned incandescent technology. While most incandescent bulbs were phased out (in Australia) years ago due to their short lifespan and high running costs, some were exempt – including those that used 25W or less. These bulbs aren’t very bright – they put out less than 250 lumens – and there are plenty of energy efficient alternatives available.

You can get the same vintage look from an energy efficient light bulb. Some vintage bulbs use LED – so it looks like it has an old tungsten wire inside, but it contains tiny LEDs. If you choose an LED version of a vintage-looking bulb, you get the look and effect you want, combined with a much longer lifespan – and will cost you far less to run.

If you want to buy a vintage light bulb, make sure you choose LED.

Directional lights


Directional lights are designed to focus on a narrow spot, such as a painting on the wall, or a slightly broader area such as a dining table. 

The glass (or opaque plastic) on a directional bulb is usually just at the front. This stops light coming out of the side or the back – so more light is directed forward. They also use a reflective surface at the back of the bulb, shaped like a satellite dish, which also helps to direct more light to the right spot. In fact, the ‘R’ in the reference code for common light bulb shapes stands for ‘reflector’, as in  the low voltage MR16 (which uses a GU5.3 cap) and PAR or R shaped bulbs which use bayonet or screw caps.

When choosing a directional lamp, always look at the beam angle stated on the packaging. Generally, a narrow beam angle is between 8-15 degrees, whereas a wider beam angle, is up to about 60 degrees.

While downlights are available in halogen, CFL and LED, a quality LED is generally the smarter choice in the long run. This is because the LED will last the longest – and cost you the least to run.

Linear (straight) and circular tubes


Linear fluorescent lamps are available as straight or circular tubes, or occasionally as a U-bend.

In recent years, linear fluorescent lamps have continued to evolve. Now, many are more compact (smaller diameter) and offer more light, using even less power – making them almost as efficient as LED. Nowadays they also last longer – almost as long as LEDs – between 8000 and 16,000 hours.

Typically they’re more common in commercial and industrial lighting installations, however their cost-effectiveness means many households use them in kitchens, garages and workshops.

Learn more about fluorescent lamps >



The Compact Fluorescent Lamp (CFL) comes in a few different shapes, such as:

  • twisted spiral
  • one, two, three or more folded tubes
  • other decorative shapes – which are less commonly found.

Generally, a CFL of a particular cap size will be a direct replacement for an old incandescent or halogen lamp. However, some CFLs will be too long or wide to physically fit in some applications. If you are replacing an old bulb, especially one concealed by a lampshade or inside an oyster lamp, check how much clearance you need around it. You may need to choose a shorter or more compressed CFL to fit.

Learn more about CFL >

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