Lawn Fertilizer Calculator

Fertilizer is sold in whole bags, so bagsNeeded rounds the raw fertilizer-weight-divided-by-bag-weight result up to the next whole bag. The calculator then works backward from that rounded bag count — actualFertilizer = bagsNeeded × bagWeight — to compute actualN, actualP, and actualK, so whenever the raw calculation doesn't divide evenly, you'll apply slightly more of all three nutrients than your target application rate called for.

The calculator only solves for the nitrogen amount you specify via applicationRateN; phosphorus and potassium are never independently targeted. Once bagsNeeded is determined from the nitrogen math, actualP and actualK are simply that bag count times the bag's fixed P and K percentages, so whatever ratio is printed on your fertilizer bag is what you get for those two nutrients — there's no way to hit a separate P or K goal without choosing a different product.

Fertilizer needed is calculated as total nitrogen required divided by the product's nitrogen percentage (as a decimal), so a 10% nitrogen product needs roughly 3.2 times more total product weight than a 32% nitrogen product to deliver the same amount of nitrogen. That directly increases bagsNeeded, and since actualP and actualK scale with bag count, a low-N, high-K product like 10-0-10 will also deposit noticeably more potassium per application than a high-N product would.

This calculator only solves for your nitrogen application rate and reports whatever P and K happen to come along at your chosen product's fixed ratio — it can't independently target a phosphorus or potassium rate. If a soil test specifies P or K needs that don't match what your nitrogen-driven bag count delivers, you'll likely need to select a different N-P-K blend, or supplement separately, rather than relying on this nitrogen-only calculation to hit all three targets.