I can't afford to purchase a refractometer, but I purchased the correct (NOT prediluted) antifreeze for my car. I just want to make sure I make the concentration properly. Is there any way to do this at home without having to purchase a measurement tool?
First of all, why are you adding coolant? If you're just doing a maintenance drain and fill, or if you're filling the first time after repairing a leak, just mix 50 percent antifreeze with 50 percent water.
If someone added water or straight antifreeze previously, there's no way to know the exact concentration without measuring it. You can buy an inexpensive tester with different color balls that float at different concentrations to give you an idea of the freeze point. They only cost two or three bucks.
Keep in mind that if you do a flush on the cooling system there will always be a lot of water stuck in the engine block that will dilute the new antifreeze. I typically add one gallon of new antifreeze, then a half gallon of water, then I run the engine to mix it up. Once the freeze point is measured you have the reservoir to add straight antifreeze or water to, to bring the freeze point where you want it.
Be aware too that those ball-type testers, or those with a pointer, use those items to compare the weight of the coolant to the weight of water. Once the mixture reaches about minus 50 degrees, if you add even more antifreeze the freeze point will actually go back up toward 0 degrees, but the indicators will float more balls or the pointer will go higher indicating a lower freeze point. As long as the freeze point keeps going down when you add more antifreeze, you're okay. Minus 35 is about the lowest you want to go. Water holds and carries more BTUs than antifreeze does, and you need that water to carry the heat to the radiator.