What are the Weight, Height, BMI etc requirements for joining the army?

Good morning George, Interesting question. So typically you will be required to be within the following BMI limits; - Over 18 years: BMI = 18 - 29.9 - 16 & 17 years: BMI = 17 - 29.9 As you currently have a BMI of 12, its unlikely you are going to be accepted, due to the arduous nature of training, and physical challenges of military life. If you're under the BMI requirements, you're more prone to injury. My advice would be to speak to a local AFCO (Armed Forces Careers Office) or even a PT at a local gym, where they can give you tailored advice on upping your weight. Typically, eating more and conducting strength training is one way to increase body weight, however I wouldn't want to give you too much information on here as I don't know the full details of yourself. As for height requirements, I believe there is a minimum height of 4ft10, however I am unsure of a maximum height. I've met soldiers who are under 5ft all the way up to near 7ft. Height requirements will vary for each role (for example, you can't be a 7ft tall helicopter pilot!). Hope that helps, and good luck with your journey.

Is it likely at all that the military would accept someone with a BMI of 15, or is a BMI of 18 the absolute bare minimum? I'd only need to gain 20lbs or so to be within the 18-29 limits.

This is a difficult question to answer on here as this would be down to the doctors discretion at selection. The above answer is the minimum standards set out, however my advice would be to apply and work on your BMI up to your selection date, should you still be bellow the standard on the day the doctor may deffer you for a period of time (you may not have to do full selection again depending on performance) and ask you to return at a later date to recheck your BMI. Hope this helps.