Fluorescent Bulb Dimensions: T5, T8 and T12 Sizes Explained

Short answer: 

Fluorescent bulb dimensions follow a T-code system in which the number after the T is the tube diameter in eighths of an inch, so T5 measures 16 mm, T8 measures 26 mm and T12 measures 38 mm. Standard lengths are 600 mm, 900 mm, 1,200 mm and 1,500 mm, with G5 caps on T5 tubes and G13 caps on T8 and T12 tubes. Since new T5 and T8 fluorescent lamps can no longer be placed on the UK market from 1 February 2024, most buyers now specify LED replacement tubes that keep the same dimensions. You can compare verified suppliers in the fluorescent and LED tube supplier directory, then work through the dimension tables, regulations, price bands and sourcing steps below. 

Key Takeaways  

• The T-code defines diameter: T5 is 16 mm with G5 caps, T8 is 26 mm and T12 is 38 mm with G13 caps. Lengths follow 600 mm, 900 mm, 1,200 mm and 1,500 mm families, with T8 tubes measuring 589.8 mm to 1,500 mm including pins under IEC 60081. 
• The UK and EU phase-out of mercury-based T5 and T8 lamps has made LED retrofit tubes the standard procurement route, and retrofit installations now dominate the LED lighting market. 
• LED replacement tubes keep fluorescent dimensions but cut energy use: a 10 W LED tube replaces a 36 W fluorescent T8, with rated lives of up to 50,000 hours on branded models. 
• UK reference prices run from about GBP 4 for a basic 600 mm T8 LED tube to GBP 18 for a 1,500 mm branded tube, with compliance governed by IEC 60081, IEC 61195, BS EN 62776 and the WEEE Regulations 2013. 

Why Do Fluorescent Bulb Dimensions Still Matter for UK Buyers? 

Fluorescent bulb dimensions are the one fixed constraint in any lighting retrofit. The market context explains why the question keeps coming up: the US and Europe LED lighting market is estimated at USD 21.6 billion in 2026, up from USD 20.55 billion in 2025, with a forecast CAGR of 5.12 percent to 2031, according to Mordor Intelligence. Retrofit projects represent 61.3 percent of LED lighting installations in 2025, and the same report attributes part of that growth directly to the EU phase-out of T5 and T8 tubes, noting that LED retrofits can lower lighting energy use by up to 80 percent. 

Because replacement tubes must fit existing luminaires, buyers need the dimension families, cap types and length tables below before they can compare suppliers or negotiate prices. 

What Do the T5, T8 and T12 Codes in Fluorescent Bulb Dimensions Mean? 

The T-code multiplies one eighth of an inch (3.175 mm) by the number after the T: T5 is five eighths, standardised to 16 mm; T8 is eight eighths, one inch or 25.4 mm, standardised to 26 mm; and T12 is twelve eighths, standardised to 38 mm. The cap base differs by family: T4 and T5 tubes use G5 caps with 5 mm pin spacing, while T8, T10 and T12 tubes use G13 caps with 13 mm pin spacing. The table below summarises the main families. 

T5 tubes are more efficient than T8 and T12 tubes, with triphosphor T5 tubes reaching around 100 lm/W against roughly 70 to 90 lm/W for T8 and about 60 lm/W for T12. T5 tubes below 16 mm diameter also require a high-frequency electronic ballast, while T8 tubes run on magnetic or electronic ballasts. 

Which Tube Lengths and Caps Fit Standard Luminaires? 

Linear fluorescent tubes come in four nominal length families. The overall length including the pins is slightly shorter than the nominal value for T8 tubes and slightly longer for T5 tubes, which is why a measurement error of a few millimetres can leave a tube that does not seat correctly. The table below lists the standard combinations defined by IEC 60081. 

Cap bases matter just as much as length: G5 caps (5 mm pin spacing) belong to T5 tubes, G13 caps (13 mm pin spacing) to T8 and T12 tubes, and both are defined in IEC 60061-1. For LED replacement tubes, the operating type matters more than the cap: tubes wired for direct mains operation require ballast removal, magnetic-ballast compatible tubes need a starter fuse replacement, and electronic-ballast compatible tubes plug into existing HF fittings without rewiring. 

Which Suppliers Sell Fluorescent-Size LED Replacement Tubes? 

Europe's LED tube supply base concentrates on manufacturers in Germany, Austria and Spain, plus specialist distributors in the Baltics and France. The LED tubes category lists 1,430 suppliers, which is why vetting against the compliance criteria in the next section matters. The table below profiles six suppliers verified on the directory, each with a company profile confirming LED tube or fluorescent lamp ranges. 

When vetting a new supplier, request the datasheet for the specific tube family, confirm the operating type matches your ballast setup, and check whether the supplier carries matching accessories such as starters, sockets and luminaires. 

What Standards and Regulations Govern Tube Dimensions and Safety? 

The dimension families themselves are standardised: IEC 60081 defines the performance specifications for double-capped fluorescent lamps including dimensions and wattages, IEC 61195 covers their safety, and IEC 60061-1 defines the G5 and G13 cap geometry. For LED replacement tubes that keep fluorescent dimensions, safety is covered by IEC 62776, published in the UK as BS EN 62776. The table below maps the relevant standards. 

On the regulatory side, the mercury exemptions in the EU RoHS Directive 2011/65/EU for general purpose linear fluorescent lamps expired on 24 August 2023 for T5 and T8 tubes and on 24 February 2023 for T12 tubes. As a result, new T5 and T8 fluorescent lamps cannot be placed on the UK market from 1 February 2024, although existing stock may still be sold and installed. Disposal of remaining tubes is governed by the Waste Electrical and Electronic Equipment Regulations 2013, which require collection by a licensed carrier because the tubes contain mercury. 

What Do LED Replacement Tubes Cost in the UK? 

UK reference prices in October 2026 for LED tubes in fluorescent dimensions vary by brand, length and operating type. The table below summarises typical price bands; actual prices depend on quantity, supplier and delivery terms. 

Efficiency data shows why the swap pays back quickly: a 10 W Philips MASTER Value LEDtube T8 delivers 1,800 lumens at 180 lm/W with a 50,000 hour rated life and replaces a 36 W fluorescent tube. Bulk orders of 100 to 1,000 pieces typically attract trade pricing below the retail bands above, so request quantity breaks and confirm packaging and lead times before ordering. 

How Do Budget and Premium LED Tubes Compare? 

Not all LED tubes in the same dimensions perform alike. The table below contrasts a basic budget tube with a premium tube, both in the 1,200 mm T8 format. 

For offices, schools and healthcare environments where flicker and colour rendering affect occupants, the premium segment is usually the right choice. For storage areas and short-cycle applications, budget tubes remain a defensible option. 

How Should Buyers Source LED Tubes in Six Steps? 

A structured sourcing process prevents mismatched dimensions and non-compliant stock. The six steps below apply to both single-site retrofits and multi-site rollouts. 

Which Questions Do Buyers Ask About Fluorescent Bulb Dimensions? 

Q: What does T8 mean in fluorescent bulb dimensions? 

A: The number after the T is the tube diameter in eighths of an inch, so T8 is eight eighths, one inch, or 26 mm. T5 is five eighths of an inch (16 mm) and T12 is twelve eighths (38 mm). The cap bases also differ: T5 tubes use G5 caps with 5 mm pin spacing, while T8 and T12 tubes use G13 caps with 13 mm pin spacing. 

Q: Can I install an LED tube in an existing fluorescent fitting? 

A: In most cases, yes. LED replacement tubes are produced in the same T5 and T8 dimensions and with the same G5 and G13 caps. Choose the right operating type: tubes wired for direct mains operation after ballast removal, tubes compatible with magnetic ballasts (starter replaced by an LED starter fuse) or tubes compatible with electronic ballasts. Safety for retrofit tubes is covered by BS EN 62776. 

Q: Are T5 and T8 fluorescent tubes still legal in the UK? 

A: Existing stock can still be sold and installed, but new T5 and T8 fluorescent lamps have not been permitted to be placed on the UK market since 1 February 2024, following the expiry of the EU RoHS mercury exemptions on 24 August 2023. Buyers should therefore specify LED replacement tubes with the same dimensions. 

Q: How should old fluorescent tubes be disposed of? 

A: Fluorescent tubes contain mercury and must be disposed of as hazardous waste under the Waste Electrical and Electronic Equipment Regulations 2013. Use a licensed waste carrier, a designated collection point or your distributor's take-back scheme, and never put broken tubes in general waste. 

Where Can You Start Comparing LED Tube Suppliers? 

Buyers who have fixed their specification can shortlist suppliers through the directory categories linked throughout this article, then request quotes in batches to compare prices and operating types side by side. 

Find verified LED tube suppliers 

Browse the LED tube directory on europages  

Sources  

1. Mordor Intelligence, "United States And Europe LED Lighting Market Size and Share Analysis", 2026: https://www.mordorintelligence.com/industry-reports/us-and-europe-led-lighting-market 
2. European Commission, "RoHS Directive", environment.ec.europa.eu: https://environment.ec.europa.eu/topics/waste-and-recycling/rohs-directive_en 
3. RS Components, "Safe Disposal of Fluorescent Lighting Tubes", September 2026: https://uk.rs-online.com/web/content/discovery/ideas-and-advice/fluorescent-tubes-disposal 
4. The Waste Electrical and Electronic Equipment Regulations 2013 (SI 2013/3113), legislation.gov.uk: https://www.legislation.gov.uk/uksi/2013/3113/contents/made 
5. Signify, "Philips LEDtube, T8, 1200 mm, 10 W, 6500 K, 1800 lm, 50000 hours", product page: https://www.lighting.philips.com/prof/led-lamps-and-tubes/led-tubes/master-value-ledtube-t8/929004230310_EU/product 
6. IEC 60081, "Double-capped fluorescent lamps - Performance specifications", IEC. 
7. IEC 61195, "Double-capped fluorescent lamps - Safety specifications", IEC. 
8. BS EN 62776, "Double-capped LED lamps designed to retrofit linear fluorescent lamps", BSI. 
9. IEC 60061-1, "Lamp caps and holders together with gauges for the control of interchangeability and safety", IEC.