Exact conversion basis
One international inch is exactly 0.0254 metre and one international foot is exactly 0.3048 metre. One square foot is exactly 0.09290304 square metre.
Convert both rise and run to the same unit before calculating a ratio. A 500 mm rise over a 1 m run becomes 500 over 1000, ratio 0.5 and pitch 6/12.
Mixed-market records
US users often lead with rise per 12 and feet or inches. UK, Canadian and Australian users may prefer degrees and metric lengths. Save the actual unit with each linear or area value instead of changing labels without converting values.
Keep a record of the original measurements, unit system, selected mode and any adjustment. Recalculate independently before ordering material or making a construction decision.
Detailed field guide
The international inch is exactly 0.0254 m and the international foot exactly 0.3048 m. Therefore 1 ft² is exactly 0.09290304 m². Area conversion squares the length factor; it is not the same numeric multiplier.
A 500 mm rise and 1 m run must first become 500 mm over 1000 mm, ratio 0.5. That equals 6/12 regardless of whether the associated project lengths are later displayed in feet or metres.
For mixed records, convert every length before addition and every area before aggregation. Keep the original source unit and converted value so another reviewer can reproduce the decision.
Round for communication only after the calculation. Converting 10 ft gives exactly 3.048 m; displaying 3.05 m is convenient but should not become the new high-precision source.
Checks before using the result
- Preserve the original measurement, unit, endpoints and drawing revision.
- Keep different pitches and unsupported intersections separate.
- Compare one representative result with an independent calculation.
- Do not access a roof merely to obtain calculator inputs.
Convert length and area differently
One international foot is exactly 0.3048 m, but one square foot is 0.3048² = 0.09290304 m². Squared units require the squared factor. Converting a plan length with an area factor, or an area with a length factor, produces a large but sometimes plausible-looking error.
Normalise mixed rise and run
A 500 mm rise over a 1 m run is not 500/1. Convert both to the same unit first: 500/1000 = 0.5, equivalent to 6/12. Pitch itself is unchanged by a consistent unit conversion because both sides of the ratio scale together.
Preserve source and converted values
Keep “10 ft source” alongside “3.048 m exact conversion.” If the display rounds to 3.05 m, label it as a display value. Reusing that rounded number as a new source creates avoidable drift, especially when many sections are added.
Use one unit for aggregation
Convert all compatible section areas into one selected area unit before summing. The workbench enforces one common unit and converts every stored field together. After conversion, check a known fixture such as 100 ft² = 9.290304 m² before relying on a large total.
Convert dimensions before forming a ratio
Pitch is dimensionless only after rise and run use the same unit. A 500 mm rise over a 1 m run is 500/1000, or 0.5—not 500/1. That ratio equals 6/12, 50 percent and about 26.57 degrees. Convert the underlying lengths first, then form the ratio. Retain the source unit in the record so another reviewer can tell whether a displayed metric or imperial value is measured or converted.
The international inch is exactly 0.0254 m and the international foot is exactly 0.3048 m. RoofFigured uses those factors and keeps full precision internally. A displayed conversion may be rounded for readability, so avoid copying the rounded display back into another field when the original measurement remains available. Repeatedly converting rounded numbers can create drift across a large multi-section record.
Remember that area uses the squared factor
Length and area do not use the same conversion number. One square foot equals 0.3048², or exactly 0.09290304 m². Therefore 100 ft² equals 9.290304 m². Applying 0.3048 directly to an area creates a large but potentially believable mistake. Use an area control for areas and a length control for lengths, and verify one known fixture whenever a record changes unit system.
When rectangular plan dimensions are converted individually, multiply the converted lengths to obtain area and compare with a direct area conversion. The two methods should agree within the documented numeric tolerance before display rounding. That independent reconciliation can expose a mistyped unit, an inadvertently rounded dimension or a field that was relabelled rather than converted.
Choose a working unit and preserve provenance
A mixed-source project may contain feet, inches, metres and millimetres. Preserve each original measurement in the supporting record, but convert all compatible working values into one selected unit before adding them. The workbench changes every stored area consistently when its area unit changes. Do not add a square-foot subtotal directly to a square-metre subtotal or compare a millimetre rise with a metre run without normalisation.
Precision should reflect the source, not merely the calculator’s ability to display decimals. A drawing dimension to the nearest inch does not become millimetre-accurate because it is shown as 25.4 mm increments. Record reasonable uncertainty, identify converted figures and round only at the presentation stage. The formulas remain exact for the supplied values, while the practical quality of the result remains limited by how those values were obtained.
Worked mixed-unit audit
Suppose a plan reports length 32 ft while a later survey reports width 8,500 mm. Convert 32 ft exactly to 9.7536 m and 8,500 mm to 8.5 m. Their rectangular plan area is 82.9056 m². An independent imperial path converts 8.5 m to about 27.887139 ft, multiplies by 32 to obtain about 892.388451 ft², then converts area with 0.09290304 to recover 82.9056 m². Agreement shows that length and area factors were applied in the right places.
At 6/12, applying the unitless factor 1.118033989 gives about 92.693 m² of ideal sloping area. The pitch factor does not need conversion because it is a ratio, but the area does. If the final record is switched to square feet, convert the unrounded area total rather than rebuilding it from rounded displayed dimensions. Preserve “32 ft source” and “8,500 mm source” in the notes so the normalised working values do not erase provenance. This matters when later drawings revise only one dimension or when a reviewer must assess the precision supported by each source.
Final verification before use
Finish a mixed-unit job by checking one length conversion, one area conversion and one dimensionless pitch. Confirm that all aggregated areas use the selected unit and that source units remain visible in notes. Recalculate from unrounded values after switching units and compare the totals through the exact conversion factor. Any difference beyond documented tolerance should be resolved before export.
Use decimal precision that the source measurement can support. Converting 10 ft gives exactly 3.048 m by definition, but a field note rounded to the nearest foot does not suddenly have millimetre-level observational accuracy. Preserve the exact conversion while recording the original resolution. This distinction allows accurate arithmetic without making an unsupported claim about measurement quality and helps explain why two independently measured values may not reconcile perfectly.
Watch compound units in product coverage. A pack covering 32 ft² does not cover 32 × 0.3048 m²; it covers 32 × 0.09290304, about 2.972897 m². Convert the coverage area and roof area to one system before division. The number of packs should remain consistent apart from rounding. If it changes materially after switching units, a length factor was probably applied to an area or a displayed rounded figure was reused.