how accurate are the bmi/height/weight machines in shops/motorway service stations

Ive always thought i was 5 foot 6 but recently ive been on 2 differant machines one at a service station and one at tesco,

They both said 5 foot 7, my friend has just measured me at 5 foot 6,
Are these bmi machines accurate, i took my shoes of for the test

I'm not sure, I used one in boots and it measured me 1 inch shorter! I felt robbed lol

Just nipped a motorway service station which is 3 minutes from me, havent tried this one b4 and it says 5 foor 7 so i think ill go for that, anyone got any ideas how to get an accurate height measure

Went doctors on friday so whilst i was there, i asked could they measure my height, im definatly 5 foot 7, he said the manual ones are alot more presise than the automatic ones,

I used one in my local Tesco recently that said I was 4ft 11 (I am actually 5ft 10)!

I find they are usually ok. If I properly stand tall each time I get the same result height wise but some times when I don't care about BMI and just want weight and slouch it really shows. It normally measures me at 5'6.6 which is my height- I'm an inbetweener Think that machine that Faxman used definately had a few issues. One of which would have been me demanding my money back!