Read the coverage basis first
Find whether the label gives square metres per litre or square feet per gallon, and whether that figure is for one coat. This planner accepts coverage per coat. If the manufacturer gives a range, use a documented value suitable for the job and test it on the actual surface; do not silently use the best-case endpoint. If the label only gives coverage for a full system, ask the supplier for a per-coat basis before entering it here. Extra allowance cannot repair an unknown coverage definition.
Keep the product schedule separate
Product A and product B are independent stages. They might be a topcoat and stain, or two different finishes used on different parts. Each has its own coverage, coat count, allowance, package size and usable stock. Select the appropriate stage on each part row. The same selected surfaces receive each checked stage on that row; create disjoint rows when the front, back or edges require different treatment. Similar colours or names do not make products interchangeable, and litres remaining from one stage cannot pay for another.
Apply the purchase arithmetic
Base litres = selected area × coats ÷ coverage per coat. Demand litres = base litres × (1 + extra allowance ÷ 100). Subtract the usable stock already available for that exact product, then divide the remaining demand by one package size and round up. The planner uses unrounded values for this last step. Displayed values are rounded for readability, so a purchase boundary should be reviewed using the inputs and exported worksheet rather than by dividing two shortened display numbers.
Check a complete worked example
For the four shelves in the area guide, selected area is 1.4832 m². At an illustrative 10 m²/L per coat and two coats, base volume is 0.29664 L. With 15% extra allowance, demand is 0.341136 L. If 0.10 L of usable matching stock is on hand, the shortfall is 0.241136 L. A 0.25 L package covers that modeled shortfall, leaving 0.008864 L across stock and the new package. These values are arithmetic examples, not a product recommendation or an application specification.
Review stock and package assumptions
Enter only stock that is usable, appropriate for the project and available to this job. Check the container, identity and manufacturer storage instructions before deducting it. One package size is modeled for each product; the tool does not search for a cheapest combination of different cans. If only a larger size is available, enter that size and recalculate. Extra allowance is an added percentage of base volume, not a spray-transfer efficiency input. Coverage, absorbency, method and practical loss still need evidence from the label and a test panel.
Record the basis with the order
The CSV includes metric dimensions, selected surfaces, product assignments, per-coat coverage, coat counts and the package calculation. Keep it with the finish schedule and label revision. The display can switch to inches, square feet and US gallons without changing the modeled physical job. An imperial gallon is a different volume and is not supported as the US-gallon input. Verify the unit printed on a supplier listing before converting a value.
Product documentation
Osmo UK: Polyx-Oil Original — The product overview lists coverage for one coat separately from application coats. Check the current variant and label; this planner does not load a brand specification as a default. Checked 2026-10-08.