Pop quiz: you want to lift 85kg. How many plates do you put on each side of the barbell, and what sizes? If you didn’t immediately answer “a 45 and a 20, of course,” then perhaps you would like a little primer on plate maths.
Yes, you count the bar
Let’s start with the basics. When you tell somebody how much weight you lifted, that includes the total poundage that you had in your hands. The bar is part of that.
At most gyms, a typical barbell is around 2 metres long and weighs around 20kg. If your gym has multiple sizes of bar, check the end for a label giving the weight, or ask somebody. Olympic style weightlifting is done with a 20 kilogram bar for men, and a shorter, narrower 15 kilogram bar for women. (The main advantage of the women’s bar, if you’re a woman, is that it’s easier to grip.)
Your gym might also have smaller training bars — five kilograms is one common weight. Home exercise equipment sometimes uses a 3 centimetre bar without the wider collars you’ll see on Olympic or powerlifting gear; those bars are lighter, and you should weigh yours or check the package to see what you’ve got.
If you’re lifting with a Smith machine, or any other kind of machine, don’t bother trying to figure out how much the machine or its bar weighs. Every machine is different, they’re almost never labelled, and the weights won’t necessarily feel the same as they would on a barbell. Just write down the total weight you loaded onto it — that’s enough for record keeping purposes.
What about the clips or collars that hold the weight on? Usually they’re not heavy enough to bother adding into your calculations, but if they’re large and you know the weight, feel free to include them.
Add ’em up
This may be obvious, but I want to make sure this information is easy to find: to find the total weight you’ve put on the bar, add up the plates on one end, double that number, and add the weight of the bar.
So if you have a 20 and an 10 on each end, add 20 + 10 to get 30, double that (60), and add the weight of the bar (60 + 20 = 80kg).
More often, though, you start with a number you’d like to lift, and then have to load the bar appropriately. Start practicing and soon you’ll be able to load a bar correctly without thinking about it too much. Here’s how:
Memorise common combinations
Let’s assume we’re working with a 20kg bar. You’ll end up using the same combinations over and over, and these numbers will start to look really familiar:

24 kilograms: a 2kg plate on each side

30 kilograms: a 5kg plate on each side

42 kilograms: a 11kg plate on each side

60 kilograms: a 20kg plate on each side

100 kilograms: two 20kg plates on each side

140 kilograms: three 20kg plates on each side
As you warm up for a lift, you can do the maths as you put plates on. Say you do a set with just the empty bar, then with a 10 on each side (that’s 40), then a second pair of 10’s (60), then swap both of those out for a 25 (now we’re up to 70) and you want to do your next set at 80. You know you need 10 more kilograms, so look for a pair of 5kg plates and there you go.
Apps and calculators help
Yes, there are calculators that will do the job for you. I track my workouts in an app called Strong, and it has a little button that will tell you how to load the bar for your lift. RackMath (free on iOS and Android) looks like a good option for a standalone app.
An online equivalent is this calculator from ExRx. Note that you have to tell any calculator how heavy your bar is, what sizes of plate you have available, and how many of each there are.
In a gym the plates may be unlimited, but in a home gym there may be certain numbers you just can’t achieve because you don’t have the right combination of plates.
Comments
Count the bar weight? Come on, really? Who does that???
I'm obviously not "getting it", assuming you're talking kilos then that answer would be 150kg.
Unless you're talking pounds I suppose, but why mix measurements 0_O
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