Chemical Mixing & Surface Area Calculator

Smooth, non-porous surfaces (like Colorsteel cladding, glass, or truck curtain sides) allow liquids to form a continuous chemical film across the surface without absorption, yielding high coverage rates (6–8 m2 per litre). Porous materials (such as unsealed concrete pavers, exposed aggregate, rough-sawn timber, and split blockwork) act like a sponge, drawing liquid deep into their pores. Porous surfaces can require two to three times more ready-to-use solution (2–3 m2 per litre) to achieve the full saturation required for the chemical to work.

Perform a simple splash test on dry ground: flick a few tablespoons of clean water onto the surface.

• If the water beads up or pools for several minutes, the substrate is non-porous.

• If the water darkens the surface and absorbs within 30 to 60 seconds, it is semi-porous.

• If the water sinks in almost instantly, it is highly porous—increase your total ready-to-use solution estimate by 30% to 50%.

• Flat Surfaces (Driveways, Decks, Slabs): Multiply Length x Width.

• Vertical Walls (Soft Washing): Measure the perimeter of the building and multiply by the wall height. For single-storey homes, an average exterior wall height is roughly 2.4 to 2.7 metres. Subtract large openings like double garage doors if you are not washing them.

• Pitched Corrugated Roofs: A roof’s actual surface area is significantly larger than the building’s ground footprint. A standard 20° to 30° roof pitch adds roughly 10% to 15% more surface area, and corrugated or standing-seam profiles add another 10% to 12% in folded metal surface. Multiply the ground footprint by 1.25 to find the true surface area to be treated.

For basic exterior residential soft washing, generally no. Most contractors keep windows and trims included in the gross square meterage because windows must be pre-wet with water, rinsed, and occasionally hit with low-pressure wash solution to remove road film and spiders. Keeping them in the total calculation also provides a built-in volume buffer.

The calculator estimates the liquid volume that must physically hit and wet the surface. Different equipment deposits volume at different speeds:

• Manual Knapsack / Backpack Sprayers: Deliver low volumes with tight droplet control, resulting in near-zero runoff. Your usage will closely align with the calculator’s lower volume estimates.

• 12-Volt Softwash Systems: Deliver high fluid flow (15–20 litres per minute). They saturate surfaces fast, but over-application and runoff are common; plan for 15% to 20% higher liquid usage.

• Pressure Washer Chemical Injectors (Downstreaming): Chemical is drawn directly through an injector valve at a fixed ratio (often 1:10 or 1:20). For downstreaming, calculate the water flow rate of your machine (e.g., 15 LPM) and dwell time rather than relying strictly on tank mix ratios.

Use a flat fan nozzle (40° to 65°) or a low-pressure hollow-cone nozzle. Avoid pinpoint jet streams (which cause localized pooling and streaking) and ultra-fine misting nozzles. Fine mists cause significant airborne drift and evaporate before soaking into the target substrate.

Real-world job sites involve environmental and operational variables that no digital formula can predict with 100% accuracy:

• Wind Drift: Gusty conditions blow fine spray off target, requiring more volume to saturate the area.

• Ambient Temperature & Sun Exposure: Hot afternoon sun evaporates chemical solutions prematurely, forcing you to re-apply to maintain the required 10- to 20-minute dwell time.

• Infestation Severity: Light green surface algae requires a light misting pass; heavy, multi-year black mould, lichen, or thick caked grease requires heavy saturation and physical agitation.

• Operator Walking Speed: Application consistency varies naturally based on how fast the operator sweeps the spray lance.

Always factor in a 10% to 15% buffer over your calculated volume. Having slightly more concentrate on hand prevents running short mid-wash, covers hose priming, compensates for wind loss, and leaves enough product for localized touch-ups on stubborn spots.

Do not simply increase the chemical concentrate beyond recommended ratios, as over-concentrated chemicals can etch soft metals, strip paint, or burn foliage. Instead:

1. Ensure the surface remains wet with solution for the full recommended dwell time (usually 10–15 minutes).

2. Agitate stubborn lichen, grease, or grime with a stiff deck broom or nylon brush while the chemical is working.

3. Apply a second pass at the standard dilution rather than one pass at a dangerously strong concentration.