If it says to use 10-10-10 on a plant.
What is the difference between 10-10-10- and 5-5-5 fertilizer?
Just use twice as much 5-5-5 or half as much of 10-10-10?

The RATIO of both products is 1:1:1. The NPK numbers on the label are the PERCENTAGE by volume in the package in (various fillers and/or other nutrients make up the remaining percentage). It's always best to follow the manufacturer's application instructions, but essentially yes, the lower percentage will require a greater volume of product to achieve whatever application strength you're aiming for.

Whether your plants actually need that ratio is another subject. This guy is annoying to listen to, but his message is valuable.
https://youtu.be/1COpSNWSG8o?s...

But anybody who is recommending a "balanced" fertilizer (where the numbers are the same) probably doesn't know much about plant nutrition or soils.

If you watched the link @lochroma, you'll see that issue is fully addressed.

Whatever. I'm not watching that video. I had 4 years of Horticulture including a year of soil science.
To the OP: If you're going to buy a fertilizer like that you might as well buy 20-20-20 as it's more bang for your buck. You could dilute to the proper concentration.

I watched it and also read the comments. He has an interesting take but may have gone a wee bit overboard in his conclusions. And a lot of his viewers don't agree (but at least they watched before deciding he was full of .... ). I want to see his credentials but can't even find his name.

Lucy, I agree. I often find him heavy-handed and overly opinionated, but often there are thruths as the basis of his wordy lectures. I listen to him at 1.75 speed.

There are other horticulture figures out there who also started out extremely knowledgeable in their given niches but got popular and turned into overbearing "experts" about all things garden-related. I believe in science, but also believe there is usually a little wiggle room when it comes to most gardening practices.