A filling factory was a manufacturing plant that specialised in filling various munitions, such as bombs, shells, cartridges, pyrotechnics, and screening smokes. In the United Kingdom, during both world wars of the 20th century, the majority of the employees were women.

In World War I, a filling factory belonging to the Ministry of Munitions was known as a National Filling Factory.

In World War II, a filling factory belonging to the Ministry of Supply was known as a Royal Filling Factory (RFF), or a Royal Ordnance Factory (ROF). These were all part of the Royal Ordnance Factory organisation, owned by the MoS.

The filling of smoke screen canisters and other pyrotechnic devices was also carried out by fireworks manufacturers, particularly in World War II, but these are not specifically covered by this article.

The filling factories' raw materials, such as TNT, RDX, or propellants, such as cordite, were manufactured in National Explosives Factories (World War I) or Explosive ROFs (World War II) and transported, by railway trains, to the filling factories for filling into munitions, produced at other plants.

High-explosives, such as TNT, had to be heated to melt them and the liquid was poured hot into heated shell cases. Care had to be taken to ensure that there were no voids in the poured explosive charge as this could lead to the shell detonating in the gun barrel during firing.

Shells and gun cartridges were manufactured in the UK, in World War II, by both the Engineering ROFs and private steel works / forging companies. It is believed that the empty brass cartridge cases for small arms were made at the Small Arms Ammunition Factories.

The filling factories produced their own pyrotechnics, such as fuses and screening smokes, as many of these were sensitive materials. They were then filled or assembled directly into the munitions.

They also would have sewn cotton bags for filling with primer composition or cordite charges.

Filling factories had a large number of buildings. Buildings were needed on the various groups for filling of munitions. Explosives magazines were required by each group to store the incoming explosive materials and to store the outgoing filled shells or gun cartridges, usually packed in ammunition boxes. Storage buildings were also needed on each group to store the incoming empty shells and the empty ammunition boxes.

For safety purposes, munitions were segregated into different compatibility groups. A World War II filling factory would generally fill several different groups of munitions; and these groups would be located in different geographical areas within the danger area of the filling factory.

The World War II groups were:

• Group 1: Initiators, such as caps and detonators for primers and fuzes.
• Group 2: Fuze pellets, exploder pellets, exploder bags.
• Group 3: Filling of fuzes.
• Group 4: Blending of gunpowders for time fuzes.
• Group 5: Filling of cartridges, such as filling cordite into cloth bags or into brass cartridge cases.
• Group 6: Manufacture of smoke producing compositions.
• Group 7: Small arms filling.
• Group 8: Filling of shells or bombs.
• Group 9: Large magazines, filled ammunition awaiting dispatch.

In addition, a filling factory would have provision for limited proofing and testing of its munitions; and burning grounds for disposal of waste explosive material.

Outside of this danger area, but still within the factory site, would be located:

• administration offices;
• pay offices;
• workshops;
• a medical centre;
• changing rooms;
• contraband storage (for items prohibited in the danger areas, e.g. matches, tobacco, etc.);
• search rooms;
• canteens (as many as 40 in some of the large factories).

At the start of the War munition filling was carried out at Woolwich Arsenal, however it was soon realised that a massive increase in production of munitions was required. A 2015 study by Historic England looked into the 170 National Factories created in England by the Ministry of Munitions in the early part of WW1 (covering 174 locations), among these were the National Filling Factories, including a National Fuse Factory, 5 Trench Warfare Filling Factories, and 3 National Factories for Filling and Assembling Chemical Shells.^{[WW1 1]} Filling of small bore ammunition was increased by the creation of 4 government cartridge factories. Most of the national filling factories followed similar designs with large sites with small lightweight buildings well separated and connected by raised walkways which had lightweight rail lines so materials could be wheeled between buildings on trolleys. Shells, fuses, packing cases, and explosives were brought in by rail to the edge of the factory, and completed munitions left by rail. All the filling work was carried out within the large 'clean area' by a large, mainly female, workforce. People entering the clean area had to change clothes and leave behind anything that could strike a spark, not just matches and lighters but all metal, even hairclips - one male worker being fined £5 for being in possession of 2 nails that he had used to replace a lost button on his trousers.^{[WW1 2]}

While some of the factories were entirely built and managed by the Ministry of Munitions, others had local management. For instance NFF 10 was built on 109 acres of land owned by White and Poppe Ltd who had changed from engine production to manufacturing fuse bodies and 18pdr shell sockets. White and Poppe was given the contract for building the fuse filling plant in September 1915, and when completed in September 1916 it ran under their management. Unfortunately a problem with fuse failures caused a cessation of production in December 1916. The fault identified and the factory reorganised it restarted production in February 1917 as NFF 21, but still under White and Poppe management.^{[WW1 3]}

When the Armistice came in November 1918, production was rapidly terminated and the female workforce laid off. A few sites, with much reduced male staff levels, were used for decommissioning unwanted ammunition. After clean up, most of the buildings, equipment, and stores were sold off by public auctions in the early 1920s.

The recruitment of workers for the factories was specifically aimed at women, and in general the workforce at the filling factories was 80 to 90% women. An advert in the papers in January 1917^{[WW1 11]} was aimed at recruiting 8000 women workers for a munitions filling factory in North-West London (Willesden Employment Exchange). They had to be aged 20 to 40, and live not more than an hour away, though a lodgings register was available. The work was generally 54 hours a week, 'but may be more', and the pay was 27 shillings, day and night shifts had to be undertaken by all workers, a fortnight of each at a time. For more information on women in munitions during WW1 see munitionettes.

Some of these filling factories were temporary "war duration" only factories and they closed after the end of World War II. Other filling factories were designed to be permanent and to remain open after the War. However, only ROF Glascoed is still open and is now part of BAE Systems.

Twenty World War II filling factories were planned, but only 16 were built. The two largest UK filling factories were:

• ROF Chorley (Filling Factory No. 1)
• ROF Bridgend (Filling Factory No. 2)

The other filling factories were:

• ROF Glascoed (Filling Factory No. 3)
• ROF Rotherwas (Filling Factory No. 4)^{[lower-alpha 1]}
• ROF Swynnerton (Filling Factory No. 5)
• ROF Risley (Filling Factory No. 6)
• ROF Kirkby (Filling Factory No. 7)
• ROF Aycliffe (Filling Factory No. 8)
• ROF Thorp Arch (Filling Factory No. 9)
• ROF Queniborough (Filling Factory No. 10)
• ROF Brackla (Filling Factory No. 11)
• ROF Swindon (Wootton Bassett) (Factory No. 12)^{[lower-alpha 2]}
• ROF Macclesfield (Factory No. 13)^{[lower-alpha 2]}
• ROF Ruddington (Filling Factory No. 14)
• ROF Walsall (Filling Factory No. 15)
• ROF Elstow (Filling Factory No. 16)
• ROF Featherstone (Filling Factory No. 17)
• ROF Burghfield (Filling Factory No. 18)^{[lower-alpha 3]}
• ROF Tutbury (Factory No. 19)^{[lower-alpha 2]}
• ROF Northampton (Factory No. 20)^{[lower-alpha 2]}

• List of Royal Ordnance Factories
• Munitionette