The percentage is equal to the amount of nitrogen in pounds in the bag. Example a 40lb bag of 20-20-20 would contain 8lbs of actual nitrogen and a 45lb bag of 30-0-4 would have 13.5 lbs of N.
Based on a general recommendation of 1lb of nitrogen per 1000sq ft for lawns the first bag would cover 8,000 sq ft and the second 13,500 sq ft.
The application rate by weight can also be figured for square footage.
Ex. 40lbs divided by 8 = 5lbs of fertilizer per 1000sq ft or Ex. 45lbs divided by 13.5 = 3.33lbs per 1000sq ft
By multiplying the percent ratio times the bag weight you get actual lbs N per bag and to divide that number by overall cost per bag you can also easily figure out the real price you are paying per lb for the fertilizer.
Ex.1 .20% x 40lbs=8lbsN @$16 a bag=$2.00lb. Ex.2 .30% x 45lb=13.5lbs @ $32 bag=$2.37lb
Most lawn fertilizers have limited amounts of phosphorus used mainly in starter blends to promote root growth they're usually not needed in established lawns. Also there is less chance of fertilizer burn with coated slow release too. When using basic field fertilizers you should time the application just before a rain so it gets watered in good.
For many years we just used a complete fertilizer on the hay field. Usually a triple 15 or better and probably worked out to about 50-60lbs per acre actual N, but doing soil samples can tell you specifically what fertilizer areas you may be lacking in and exactly how much you need to apply and can also save money by knowing what ratio blends to get. Nitrogen for top growth P and K for root structure and strength.
