E12, E14, SES, E17 – all fingers and thumbs

ES and SES measurer

11w ES T2 helix

11w ES T2 helix

We are talking about lamp caps.  ‘Lamp caps’ are the part of a light bulb which makes the electrical connection with your light fitting.

These can often be described as BC (bayonet) or ES (Edison Screw developed by Thomas Edison), but in the case of screw-in bulbs there are quite a few similar names or codes for bulbs which are very similar – but get the wrong lamp cap and the bulb simply won’t fit!

So what is the difference and what do all the codes and numbers mean?

A ‘standard’ ES (Edison screw) bulb is also known as E27, which you might see on some light bulb boxes.  This is the diameter of the screw thread, i.e. 27mm in diameter.

The only larger screw fitting that you are likely to see is in some high-bay fittings (e.g. a barn or warehouse).  They use E40/GES (Giant Edison Screw) bulbs, which are 40mm in diameter, but they aren’t normally used in the home.

The other most common screw-in bulb for the home is the E14 or SES (Small Edison Screw).  This, as you will have correctly guessed has a 14mm diameter screw cap.

ES and SES measurer

ES cap, width of your thumb. SES, width of your little finger.

I don’t have a ruler handy…

Ok, the easiest way to tell an E14/SES bulb from an E27/ES bulb is:

Other screw-cap bulbs

Other screw capped bulbs that you might be looking for, especially if you need a replacement pygmy bulb are:

Table of Edison Screw-cap fittings

Designation Base Diameter
(Crest of thread)
Name Usage
E5 5 mm Lilliput Edison Screw (LES) Very uncommon
E10 10 mm Miniature Edison Screw (MES) e.g. torch bulbs
E11 11 mm Mini-Candelabra Edison Screw (mini-can) Very uncommon
E12 12 mm Candelabra Edison Screw (CES) Candelabras, boats and some Christmas decorations
E14 14 mm Small Edison Screw (SES) Common household bulb, pygmy version used in inflatable Santas
E17 17 mm Intermediate Edison Screw (IES) e.g. microwaves
E26 26 mm [Medium] (one-inch) Edison Screw (ES or MES) Very uncommon
E27 27 mm [Medium] Edison Screw (ES) Common household screw-in bulb
E29 29 mm [Admedium] Edison Screw (ES) Very uncommon
E39 39 mm Single-contact (Mogul) Giant Edison Screw (GES) Very uncommon
E40 40 mm (Mogul) Giant Edison Screw (GES) High-bay barns and warehouses

We recommend measuring the screw thread of the bulb (preferably with calipers) to work out which type of bulb you need.

For much more information about Edison screw bulbs, try this Wikipedia article: http://en.wikipedia.org/wiki/Edison_screw


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