TryBuildCalcWeed and Feed Calculator (Combination Herbicide + Fertilizer)
Size weed and feed for your lawn instantly.
🕒 Last updated: September 21, 2026
Inputs
ℹ️Total lawn area to be fertilized.
Application Scenario
A light application as the lawn breaks winter dormancy — commonly cited in the 0.5-1 lb N/1,000 sq ft range. Pushing a heavier rate this early can force excessive top growth before the root system has fully reactivated.
Fertilizer Product
ℹ️N-P-K ratio sets how much of the product's weight is actual nitrogen (N) — a higher N percent means fewer lb of product needed for the same nitrogen dose. Select Custom N-P-K Ratio to enter your own product's exact label ratio.
A combination herbicide + fertilizer product. These carry their own timing constraints this calculator does not model: a pre-emergent weed & feed must go down before weed seeds germinate (and will also prevent grass seed from germinating), while a post-emergent weed & feed needs actively growing weed foliage to work and can injure grass seedlings or newly laid sod. Never use a weed & feed product at seeding, overseeding, or sodding.
Check your product's label for terms like "slow-release," "controlled-release," "coated," or "WIN" (water-insoluble nitrogen). This cannot be determined from the N-P-K ratio alone — Purdue Extension's higher single-application safe-nitrogen threshold only applies when roughly half or more of the product's nitrogen is slow/controlled-release.
Fertilizer Bag Size
ℹ️Common retail bag sizes — match your specific product's actual bag weight, or enter a custom size.
Cost
Fertilizer Needed
15.625 lb
7.087 kg
Bags to Order
1
20 lb (≈9.1 kg) — standard bag
Lawn Area & Application
Lawn area: 5000 sq ft (464.5152 m²)
Scenario: Spring Green-Up
Target nitrogen rate: 0.75 lb N / 1,000 sq ft
Nitrogen needed: 3.75 lb (1.701 kg)
Fertilizer Product & Bags
Product: Weed & Feed, Combination (24-0-4) (N-P-K 24-0-4)
Bag size: 20 lb (9.07 kg)
Bags needed: 1
Delivers approx. 0 lb P2O5 and 0.625 lb K2O along with the nitrogen above.
Assumptions Used
The target nitrogen rate used (A light application as the lawn breaks winter dormancy — commonly cited in the 0.5-1 lb N/1,000 sq ft range. Pushing a heavier rate this early can force excessive top growth before the root system has fully reactivated.) is a commonly cited reference figure for the Spring Green-Up scenario — actual recommended rate varies by grass species, climate, and the specific product's own label, so a specific product's label guidance should be used when available (enter its ACTUAL nitrogen rate, not its product application rate, via the "Use Custom Target Rate" option above). A combination herbicide + fertilizer product. These carry their own timing constraints this calculator does not model: a pre-emergent weed & feed must go down before weed seeds germinate (and will also prevent grass seed from germinating), while a post-emergent weed & feed needs actively growing weed foliage to work and can injure grass seedlings or newly laid sod. Never use a weed & feed product at seeding, overseeding, or sodding. This calculator estimates fertilizer product quantity only — it does not verify soil test results, local phosphorus-use restrictions, or a full-season feeding schedule.
Diagram simplified for clarity (not to scale) — lawn area shown schematically, fertilizer coverage shown illustratively.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Lawn Fertilizer Calculator.
Weed and feed quantity — with timing caveats
A weed & feed product combines an herbicide with fertilizer in one application, but carries its own timing constraints this calculator surfaces explicitly: a pre-emergent formula must go down before weed seeds germinate (and will also prevent grass seed from germinating), while a post-emergent formula needs actively growing weed foliage to work.
This page defaults to a 5,000 sq ft lawn getting a Spring Green-Up weed & feed application — this calculator will block this product specifically if you switch the scenario to New Lawn/Starter, since a combination herbicide product is not safe to use at seeding, overseeding, or sodding.
Lawn Fertilizer Calculator Formula: How Is It Determined?
Lawn area combines with a target nitrogen rate (set by application scenario or a custom override) into a total nitrogen dose, which converts into a product weight using the product's N percent, then into a bag count.
Step 1 — Lawn Area
By Dimensions: Area = Length x Width
By Total Area: Area = entered value (converted to sq ft if entered in m²)
Either enter a rectangular lawn's length and width, or enter its total area directly — useful for an irregular lawn where length x width doesn't apply.
Step 2 — Nitrogen Needed
Nitrogen Needed (lb) = (Area / 1,000) x Target N Rate (lb per 1,000 sq ft)
Each application scenario has its own commonly-recommended target nitrogen rate — or enter your specific product's ACTUAL nitrogen rate directly with the Custom Target Rate option (convert a bag's product application rate to actual N by multiplying it by the product's N% ÷ 100 — a bag label typically states lb of PRODUCT to apply, not lb of nitrogen).
Step 3 — Product Weight & Bag Count
Product Needed = Nitrogen Needed / (N percent / 100)
Bags Needed = ceil(Product Needed / Bag Size)
A higher-N product needs fewer lb of product for the same nitrogen dose. Product weight is calculated in lb internally (with a kg figure always shown alongside), then divided by your selected bag size and rounded up to the next whole bag.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Lawn Area | 5000 sq ft (464.5152 m²) | Sets total nitrogen dose |
| Application Scenario | Spring Green-Up | Sets the target nitrogen rate |
| Fertilizer Product | Weed & Feed, Combination (24-0-4) (N-P-K 24-0-4) | Sets the product-weight divisor |
| Bag Size | 20 lb (≈9.1 kg) — standard bag — 20 lb | Sets the bag-count divisor |
Step 1 & 2 — Area & Nitrogen Needed
| Calculation | Result |
|---|---|
| Lawn Area | 5000 sq ft (464.5152 m²) |
| Nitrogen Needed = (5000 / 1,000) x 0.75 lb N | 3.75 lb (1.701 kg) |
Step 3 — Product Weight & Bags
| Calculation | Result |
|---|---|
| Product Needed = 3.75 lb N / (24% N / 100) | 15.625 lb (7.087 kg) |
| Bags = ceil(16 / 20 lb per bag) | 1 bag |
Figures above are rounded for display; the bag count is always calculated from full, unrounded values, so it can occasionally look off by one bag if you redo the division yourself using the rounded figures shown.
Therefore, a 5000 sq ft lawn getting a Spring Green-Up application with Weed & Feed, Combination needs approximately 15.625 lb (7.087 kg) of fertilizer, or 1 bag.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.