Looking through thread assortment at our local Warmart, there didn't appear much diffetence between general purpose, embroidery or upholstery thread. Some was polyester, some polyester blend or nylon.  

What is most durable thread for hand sewing repair of backpacks, web gear, sleeping bags, tarps, heavy clothes, etc? Are there different diameters or pounds test for thread?

Thanks

I'm not much of a tailor, but I have used sailcloth thread to make some repairs. I just happened to have some left over from when I worked on a sailboat one summer. You could also look into the type of thread used for motorcycle jackets, I was told it is an aramid (Kevlar) and a lot stronger than most consumer sourced thread.

Not much for help, but it is a direction to try.

I've used spiderwire fishing line with stupidly good results on some things. The nice thing, I always have multiple spools of different line lying around.

For the odd job that comes up I have used dental floss several times when it has to be tough.  Probably have it on hand already.  White, or red and green (mint and cinnamon) if you need color match.

Probably not going to find the type/color of thread you're looking for at WalMart.

Suggest you do a search on mil-spec sewing thread and go from there.

Generally, use heavier thread on heavy items, lighter thread on light items.  Composition of the thread (material) is a component of its' strength, as well as the thread diameter.  Try to match the thread (both in diameter and material) to the item being sewed, unless there is good reason for not doing so.

What you WILL need is appropriate needles, some of which are very stout.  In some cases, you night need to use a sharp awl to punch a pilot hole in thick fabric to allow the needle to penetrate.  You will also need a properly-fitting thimble, preferably a stout (thick) one to help push the needle through some the fabric; the thimble is NOT a substitute for the awl.  Countycomm sells decent titanium thimbles.

Some decent needlenose pliers can also be useful.  Suggest not cheaping out on the needle nose pliers, as a decent pair can be useful for many other things.  Once you have used a decent pair of parallel-jaw pliers, you will treasure them: https://smile.amazon.com/gp/product/B01FSVMGXY/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Something like this: HD Needle Kit

might serve, as well as a selection of smaller sewing needles for use on lighter fabrics.

#69 Nylon thread (the military calls it "Ticket E").  

If it's going to be exposed to ultraviolet sunlight and seawater an alternate is Polyester #69.

You can find it at Wallymart or Joann's as upholstery thread.

Also suggest that if OP is sewing through fabric that is coated on one side (for water-proofing) then some brush-on sealer will help prevent water ingress through the new holes in the fabric: Seam Sealer

As an aside, I have used this chem on many different items, such as packs, bags, pouches, and Gen 1 USGI ECWCS Rain garments in order to diminish the amount of water ingress into the items caused by the many "sew-throughs" of the coated fabric.  It works; not perfect, but an undeniable improvement over the original (leaky) state.

I've used "extra strong Upholstery thread" it usually has a waterproof coating for marine use.

Your local hobby lobby or craft store should have it

Hobby Lobby

I use it for sewing arms legs and heads on my kids stuffed animals I make him. It's strong stuff.

As Sinister says, 69 or E thread,
As a skydiver, rigger, and gear builder, I have used miles of it.
Both in machines and for hand tacking.

I really like artificial sinew:



It's just heavily waxed nylon like dental floss. It's typically flatter, but it can be splint into much smaller threads. It's also UV and moisture resistant.

ROCK6