Starlight Tools

Lawn Fertilizer Runoff Risk Calculator

Screen a planned or recent lawn-fertilizer application for runoff risk. The tool combines fertilizer management with rain, ground, slope, turf, and drainage-path conditions, then prioritizes practical ways to reduce risk.

Private: all calculations run in your browser. Inputs are not sent anywhere.

Application and site inputs

1. Fertilizer amount
ft²

lb

lb

For this same lawn area, from the product label or local recommendation.

Enter the three numbers from the product analysis, such as 24-0-6. The middle number is phosphate, not elemental phosphorus.

2. Source management

Never hose fertilizer into a street or storm drain.

3. Rain or irrigation event
hours

in

Use the total expected event depth. Enter 0 if none is expected.

4. Runoff pathway

A buffer does not override local setbacks or a direct concentrated-flow path.

Screening tool only. Follow the fertilizer label and applicable fertilizer, water-quality, and setback rules. If fertilizer has already entered a storm drain or waterway, use local spill-reporting guidance.

Runoff risk screen

Enter the application and site conditions, then calculate.

How the runoff risk score works

This is a transparent screening index, not a calibrated runoff or nutrient-loss model. It follows the source-and-transport concept used in nutrient management: nutrients must be available at the surface, and runoff must have a path to carry them away.

Total score (0–100) = rate versus recommendation (0–15) + phosphate need (0–10) + pavement cleanup (0–15) + ground condition (0–15) + near-term water event (0–15) + slope/drainage (0–10) + turf cover (0–8) + connection to a drain or water (0–12).

Scores are grouped as low (0–19), moderate (20–39), high (40–59), or very high (60–100). Frozen or snow-covered ground, saturated ground with near-term water, or fertilizer left on pavement before near-term water can raise the result to a very-high minimum. These conservative floors reflect conditions in which normal soil and vegetation retention may be bypassed.

The nutrient math is separate from the risk index:

nutrient applied = product amount × label percentage ÷ 100
elemental phosphorus = label P₂O₅ amount × 0.4364

Important limit: the calculator does not turn the score into pounds or kilograms lost. Actual loss depends on fertilizer chemistry, rainfall intensity and duration, infiltration, antecedent moisture, microtopography, plant uptake, erosion, drainage networks, and maintenance.

A practical pre-application check

  1. Read the label. Confirm the analysis, rate, watering directions, spreader setting, and restrictions.
  2. Check need and timing. Use current soil-test and local turf guidance; avoid applying before a forecast storm or to frozen, snow-covered, saturated, or ponded soil.
  3. Break the transport path. Sweep hard surfaces, keep fertilizer away from drains and water, maintain dense turf or a vegetated buffer, and prevent irrigation from running off.
  4. Recheck after changes. Adjust the calculator inputs to see which feasible change most reduces the screening score.

Sources, assumptions, and review

Calculator version: 1.0. Implemented by: Starlight Robotics. Last technical content review: August 3, 2026. No field calibration, laboratory validation, or site-specific professional review is claimed.

The point weights and category boundaries are Starlight Tools screening assumptions built from the cited factor groups; they are not an EPA, NRCS, or extension-service rating system. Local laws and product labels may be stricter.

Frequently asked questions

Can this predict how much fertilizer will reach a stream?

No. It calculates nutrients applied and screens runoff conditions, but does not estimate nutrient mass lost. Site-specific monitoring or a validated local model would be needed for that.

Should I fertilize before rain?

Follow the label and local guidance. Do not apply before a forecast storm or when runoff is likely. Label-directed light irrigation is different from uncontrolled heavy rain, ponding, or irrigation that leaves the lawn.

Why does fertilizer on pavement add so much risk?

Pavement provides no soil or turf to retain nutrients and may connect directly to a drain. Sweep granules back onto the lawn or collect them; never hose them into the street.

Why ask whether phosphate is needed?

Phosphorus contributes to surface-water nutrient pollution, and some jurisdictions restrict phosphorus lawn fertilizer except for soil-test need or lawn establishment. Check the label and local rules.

Is slow-release fertilizer runoff-proof?

No. Any fertilizer can be misplaced or transported. Rate, timing, placement, ground condition, and the runoff path still matter.

What counts as a vegetated buffer?

A stable strip of dense vegetation between the treated lawn and receiving drain or water. Width, slope, concentrated flow, vegetation, and maintenance affect performance, and local setback requirements still control.

Does a low score mean an application is permitted?

No. It means fewer risk factors were selected in this screen. It does not interpret the product label or national, state, provincial, or local law.

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