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In 2026, there were just under 137,000 regular personnel serving in the British Armed Forces, compared with 151,000 ten years earlier. In the first half of the twentieth century, there were two huge spikes in the number of active personnel, which represented the final years of World War One and World War Two, numbering 4.58 million and 4.69 million in 1918 and 1945, respectively. Ever since 1945, the size of the regular armed forces has been in almost constant decline, with the noticeable exception of the early 1950s, when the number of active personnel increased by almost 200,000 because of the Korean War. 

The winds of change

There are several reasons why the number of personnel in Britain’s armed forces has declined. Britain is involved in a far fewer conventional military conflicts today than it was in the past. As the size of Britain’s empire declined rapidly after 1945, so too did the UK's global military commitments. There are also more recent developments, such as the UK government’s Strategic Defence and Security Review of 2010, which outlined personnel would be cut throughout the 2010s to modernize the UK’s armed forces. Recent geopolitical events such as the War in Ukraine have, however, led to commitments from the UK government to spend more on defence, and eventually even a reversal of personnel cuts.

Branches of the UK military

There are three main branches of the UK armed forces, the British Army, the Royal Navy, the Royal Air Force, and the Royal Marines. Of the 181,000 people serving in the UK’s armed forces in 2025, over half of them were in the British Army, which had around 109,000 personnel. The next largest branch was the Royal Navy at 37,900, followed by the RAF at 34,800 personnel. The average age for people serving in the armed forces was 31 in 2024, with the Royal Air Force having a slightly higher average age group than the other branches, at 33.