Walk into any electrical wholesaler or search online for a replacement fluorescent tube and you will encounter codes like T8 G13, T5 G5, or T4 2-pin. These codes encode tube diameter, length, cap type, and colour temperature — all the critical parameters you need to select a compatible replacement. This guide decodes every part of the designation.
Part 1: The “T” Number — Tube Diameter
The “T” stands for tubular. The number that follows it is the tube diameter expressed in eighths of an inch.
| Code | Diameter (imperial) | Diameter (metric) |
|---|---|---|
| T4 | 4/8 inch | ~12.7 mm |
| T5 | 5/8 inch | ~15.9 mm |
| T8 | 8/8 inch (1 inch) | ~25.4 mm |
T4 Tubes
T4 is the slimmest common fluorescent format. Most T4 tubes use a 2-pin G4 or G5 cap and are found in under-cabinet striplight fittings, bathroom mirror lights, and compact under-shelf lighting.
The T4 20W 565mm 6400K Daylight Fluorescent Tube is a 2-pin T4 in the full 565 mm length at 6400 K daylight colour — a common size in kitchen under-cabinet and bathroom fittings.
T5 Tubes
T5 is widely used in office luminaires, retail display cabinets, and task lighting systems. They use a G5 (2-pin miniature) cap and are not backwards-compatible with T8 fittings. Standard lengths: 300 mm (8 W), 550 mm (14 W), 850 mm (21 W), 1,150 mm (28 W).
- T5 8W 300mm G5 Fluorescent Tube 2700K Warm White — pack of 4
- T5 8W 300mm G5 Fluorescent Tube 2700K Warm White — pack of 2
T8 Tubes
T8 is the most widely installed fluorescent format in commercial and industrial environments worldwide. The 25 mm diameter accommodates a G13 bi-pin cap and is used in standard 2 ft (600 mm) and 4 ft (1,200 mm) batten fittings.
- Philips T8 18W 2ft 600mm 3500K G13 Fluorescent Tube — the definitive commercial-grade 2 ft tube in white (3500 K)
Part 2: The Cap Code — What Goes into the Fitting
| Cap Code | Pins | Common Format | Notes |
|---|---|---|---|
| G5 | 2-pin, 5 mm spacing | T5 mini | Under-cabinet, display |
| G13 | 2-pin bi-pin, 13 mm spacing | T8 standard | Office, industrial, retail |
| G4 / 2-pin | 2-pin, 4 mm spacing | T4 | Bathroom, kitchen rail |
| 2G11 | 4-pin, square base | PL-L / twin-tube | Architectural downlights |
| GR8 / 2-pin | 2-pin flat bridge | 2D lamp | Corridor, stairwell bulkhead |
This is where most ordering errors happen. A T8 tube with a G13 cap will not fit a T5 G5 fitting regardless of length. Always verify both the tube type and the cap type before ordering.
Part 3: The Colour Code — Reading the Three-Digit Number
First digit — Colour Rendering Index (CRI) class
| First digit | CRI band | Typical use |
|---|---|---|
| 8 | 80–89 (Good) | General commercial, retail |
| 9 | 90–99 (Excellent) | Graphics, colour matching, medical |
Last two digits — Correlated Colour Temperature (CCT) × 100 K
| Code | CCT | Appearance | UK common name |
|---|---|---|---|
| 27 | 2700 K | Warm, amber | Warm white |
| 30 | 3000 K | Slightly warm | White |
| 35 | 3500 K | Neutral | White / natural |
| 40 | 4000 K | Cool neutral | Cool white |
| 65 | 6500 K | Bluish | Daylight |
So a tube marked 835 means CRI 80–89, 3500 K neutral white. The Philips T8 18W 600mm 3500K (835) carries exactly this designation.
Worked Example: Replacing a Tube in a Commercial Batten Fitting
You have a 2 ft ceiling batten; the tube is marked Philips TLD 18W/835. Decoding:
- TLD = T8 tube, linear, dimmable
- 18W = power consumption
- 835 = CRI 80+, 3500 K neutral white
- Cap = G13 (standard T8 bi-pin)
Replacement: Philips T8 18W 2ft 600mm G13 3500K.
Summary
Getting a fluorescent tube replacement right comes down to three parameters: diameter (T number), cap type (G code), and colour designation (three-digit code). Match all three to your existing tube and the replacement will be plug-and-play.
Our full range of T4, T5, and T8 tubes is in stock and ships with free standard UK delivery.