Calculation methodology and checks
How the calculators reach their results, which assumptions they use and how you can check a result yourself. This page is written from the calculators' actual code; where calculators work differently, each one is described separately.
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Geometry, consumption estimates and default values
Every result combines three kinds of data with different reliability.
- Geometric calculation – area, perimeter and volume from the dimensions you enter. This is plain maths: it is as accurate as your measurements.
- Consumption estimate – the amount of material per m² or m³ (consumption rate), e.g. kilograms of adhesive per m² of tiles. It is an average for usual conditions; real consumption depends on the product, the substrate and the installer.
- Recommended default values – settings you can change (reserve, layer thickness, pack size, number of coats). They are a starting point for the estimate, not a rule.
Where formulas and consumption rates come from
The geometric formulas are standard (rectangle, cylinder, room perimeter). The consumption rates are common values for an approximate calculation. The rates in the tile, skim coat and paint, and self-levelling screed calculators are not taken from a specific product's data sheet, so they should not be treated as manufacturer data.
Where a calculator lets you enter consumption or pack size (e.g. facade, plastering, tile packaging), enter the value from the data sheet and label of the product you are buying. An overview of values by material is on the Standards page.
Units and conversions
Internally the calculators work in metric units (m, m², m³, kg, L). If you choose another unit system in the settings, your entries are converted to metric before the calculation and the result is converted back only for display – the formula stays the same.
Measurement accuracy has the biggest effect on the result: an error of a few centimetres per dimension carries through into area and volume. The number of decimals is for display; rounding up to whole pieces, kilograms and packs is described in the table below.
Openings, layers, thickness, reserve and packs by calculator
The calculators do not follow one universal procedure. The table describes the calculators whose procedure has been checked in the code for this page; the other calculators state their assumptions on their own page.
| Calculator | Openings, layers and thickness | Reserve and rounding | Packs and offcuts |
|---|---|---|---|
| Ceramic Tiles | Surfaces are entered as width × height (an area can also be transferred from the tile planner). Openings are not deducted automatically – enter surfaces without openings or split them into parts. | The reserve is a factor on the area: +5 %, +10 % or +15 % depending on the laying pattern. Pieces are rounded up; adhesive (5 kg/m², 6 kg/m² for tiles of 60 cm and larger) and grout are calculated from the area including the reserve and rounded to whole kilograms. | Boxes are shown only if you enter the packaging from the label (m² or pieces per box); no box size is assumed. Adhesive in 25 kg bags, grout in 5 kg packs. |
| 2D Flooring Layout Planner | The layout is drawn on the real room shape, including columns; cut pieces are counted one by one. | The reserve is a percentage of the number of pieces (default 10 %), rounded up. The waste share is also shown: (area bought − area tiled) / area bought. | Every cut piece counts as a new tile. The exception is board laying with offcut start, where the offcut from the previous row is counted. |
| Paint, Putty & Plasterboard | Walls = perimeter × room height − the entered area of windows and doors; ceiling = length × width. A known wall and ceiling area can also be entered. | No extra reserve: skim coat 1.5 kg/m² (for two coats), paint 0.125 L/m² per coat, 2 coats. Quantities are rounded up to whole kilograms and litres. | 25 kg bags of skim coat and 15 L buckets of paint are rounded up; the difference to a full pack is surplus. |
| Self-Leveling Compound | Room area (length × width) or an entered area; layer thickness in millimetres. | No extra reserve: 1.7 kg/m² per millimetre of thickness, rounded up to whole kilograms. | 25 kg bags are rounded up. |
| Concrete | Box (slab, footing, beam), cylinder (round column or footing) and stairs as a wedge (half a box). Openings are deducted only for slabs, through the full thickness. | Reserve 5 %, 10 % or custom, then rounding up to 0.01, 0.1 or 1 m³. Mass is estimated from the volume including reserve and a density of 2400 kg/m³. | The volume to order is the rounded volume including reserve; for site mixing, cement in bags and aggregate are shown for the selected concrete class. |
| Facade (ETICS) | Openings are deducted from the gross wall area, and reveals are added according to depth and the selected sides of each opening. The plinth is calculated separately. | Each item has its own reserve (default 5 %): the quantity needed is the net quantity increased by the reserve. | Packs are rounded up; the surplus (bought − needed) is shown separately from the quantity needed. |
| Plastering calculator | Openings are deducted if they are larger than the threshold you set (by default all are deducted); the reveal area can be added. Layer thickness is set separately for walls and ceiling. | Reserve per material type (default 5 %), then rounding to whole bags or buckets. | Pack surplus is shown separately from the quantity needed; corner beads are calculated from the required lengths and the bead length. |
| Cutting plan | You enter the pieces to be cut and the lengths of bars or boards in stock. | Saw kerf and end trimming of new bars are taken into account; the result is a cutting layout and the number of bars to buy. | Existing offcuts are used only if you enter them as stock. A remainder longer than the set minimum length is shown as a usable offcut, the rest as waste. |
Quantity needed and quantity to buy
The quantity needed is calculated from the area or volume and the consumption rate, including the reserve if the calculator uses one. The quantity to buy is that value rounded up to whole pieces, bags, buckets or boxes. The difference between them is pack surplus – which is not the same as waste on site.
Neither value is the quantity actually used. Real consumption is known only after the work is done and depends on the substrate, tools and working method.
Calculation steps and variant comparison
Most calculators (e.g. tiles, levelling compound, skim coat and paint, concrete, rebar, epoxy, anchors, levelling clips, microcement and volume) show “How was this calculated?” below the result: the steps with formulas and intermediate results, computed by the same function as the result. Intermediate results are rounded for display only.
The tile, skim coat and paint, and levelling compound calculators also compare variants: A is the current input and B the same calculation with changed parameters (e.g. size, thickness or pack). The difference B − A is shown, without prices.
Packs by market
The default bag or bucket size depends on the user’s market: the measurement system in settings and the device region. For example, in the USA tile adhesive is offered in 50 lb bags and cement in 94 lb bags, while in Europe a bag of adhesive is 25 kg. The required amount in kg or L is the same for every size; only the number of packs and the surplus in the last pack change. A size is changed only where the typical size is confirmed by a source from that region, and a saved calculation keeps the size it was saved with. The examples on this page use metric defaults.
When offcuts are reused
- The tile calculator and the tile planner do not count reuse of offcuts from cut tiles: every cut piece counts as a new tile, so the purchase is conservative.
- The exception in the planner is board laying with offcut start: each new row starts with the piece cut off in the previous row, and that offcut is counted in the purchase.
- The cutting plan uses existing offcuts only if you enter them as stock; they are then used before new bars.
- Calculators based on consumption rates (skim coat, paint, self-levelling screed, concrete) do not track leftovers – pack surplus stays unused.
Limitations and when you need manufacturer data or a design
- Results are estimates for planning and purchasing, not a substitute for a design, structural calculation or bill of quantities.
- For a specific product, the manufacturer's data sheet and instructions are decisive (consumption, layer thickness, drying time).
- Load-bearing structures, thermal insulation and building services require a design by a qualified engineer; the calculator estimates quantities and does not size the structure.
- The calculators as a whole are not certified or professionally audited and do not guarantee compliance with regulations.
- Uneven substrates, absorbency, working technique and breakage can change real consumption compared with the calculation.
Control examples you can check yourself
Each example lists the inputs with units, the procedure, the result before reserve and rounding, and the final result. You can enter the same inputs in the calculator and compare the result.
Self-levelling screed: from layer thickness to number of bags
The simplest case: a rate per millimetre of thickness, no reserve, rounding to kilograms and to bags.
| Item | Value |
|---|---|
| Inputs | |
| Area | 18 m² |
| Layer thickness | 4 mm |
| Procedure | |
| Consumption per m² | 1.7 kg/(m²·mm) × 4 mm = 6.8 kg/m² |
| Mass | 18 m² × 6.8 kg/m² = 122.4 kg |
| Rounding to whole kg | 122.4 kg → 123 kg |
| Result before reserve and rounding | |
| Mass | 122.4 kg |
| Final result (to buy) | |
| Mass | 123 kg |
| Bags | 5 × 25 kg = 125 kg |
Manual check: the steps in the table can be repeated on paper or with an ordinary calculator; intermediate results are rounded for display.
Automated check: every time the code is checked, the same example is calculated with the function the calculator uses, and the test fails if a published number differs from the result.
Wall tiles with a reserve for complex laying
The order of the calculation: the reserve is added to the area, pieces are rounded up, and adhesive and grout are calculated from the area including reserve.
| Item | Value |
|---|---|
| Inputs | |
| Surface (width × height) | 2.40 m × 2.50 m |
| Tile size | 60 × 30 cm |
| Joint width | 2 mm |
| Reserve (complex laying) | +15 % |
| Procedure | |
| Area | 2.40 × 2.50 = 6.00 m² |
| Area including reserve | 6.00 m² × 1.15 = 6.90 m² |
| Area of one tile | 0.60 × 0.30 = 0.18 m² |
| Number of tiles | 6.90 / 0.18 = 38.33 → 39 pcs |
| Adhesive | 6.90 m² × 6 kg/m² (≥ 60 cm) = 41.4 kg → 42 kg |
| Grout per m² | 2 × (60 + 30) / (60 × 30) × 1.6 = 0.16 kg/m² |
| Joint width | 6.90 m² × 0.16 kg/m² = 1.10 kg → 2 kg |
| Result before reserve and rounding | |
| Area | 6.00 m² |
| Number of tiles | 6.00 / 0.18 = 33.33 pcs |
| Final result (to buy) | |
| Number of tiles | 39 pcs |
| Adhesive (bags) | 2 × 25 kg = 50 kg |
| Grout (packs) | 1 × 5 kg = 5 kg |
Manual check: the steps in the table can be repeated on paper or with an ordinary calculator; intermediate results are rounded for display.
Automated check: every time the code is checked, the same example is calculated with the function the calculator uses, and the test fails if a published number differs from the result.
Concrete for round columns
Cylinder volume, a percentage reserve and rounding up to the selected step.
| Item | Value |
|---|---|
| Inputs | |
| Columns | 4 pcs, Ø 0.30 m, h = 2.80 m |
| Reserve | 10 % |
| Volume rounding | 0.01 m³ |
| Procedure | |
| Cylinder volume | π × 0.15² × 2.80 × 4 = 0.792 m³ |
| Volume including reserve | 0.792 × 1.10 = 0.871 m³ |
| Rounding up | 0.871 → 0.88 m³ |
| Mass | 0.871 m³ × 2400 kg/m³ ≈ 2090 kg |
| Result before reserve and rounding | |
| Volume | 0.792 m³ |
| Final result (to buy) | |
| Volume to order | 0.88 m³ |
| Mass | ≈ 2090 kg |
Manual check: the steps in the table can be repeated on paper or with an ordinary calculator; intermediate results are rounded for display.
Automated check: every time the code is checked, the same example is calculated with the function the calculator uses, and the test fails if a published number differs from the result.
What is covered by automated tests – and what is not
The project has automated tests that run selected calculation cases and compare them with expected values. The tests check the calculation logic, not the whole interface, the PDF or every combination of inputs, and they do not replace a professional review.
In addition, the automated code check (CI) runs end-to-end tests on the web version in the Chromium browser: every calculator in the registry produces a result, PDFs download for selected calculators, and switching language and resuming a draft are checked. For now they do not block a release and do not cover the Android app or ads.
Calculators whose calculation logic has automated tests:
- Ceramic Tiles
- 2D Flooring Layout Planner
- Levelers
- Paint, Putty & Plasterboard
- Self-Leveling Compound
- Microcement calculator
- Concrete
- Rebar
- Facade (ETICS)
- Anchors
- Plastering calculator
- Masonry
- U-value and insulation
- Drywall calculator
- Stone carpet calculator
- Epoxy floor calculator
- Decking calculator
- Fence calculator
- Roof calculator
- Solar PV system
- Air conditioner size
- Underfloor heating
- Excavation and fill
- Waterproofing
- Cutting plan
- Staircase Calculator
- Surface Areas
- Volume
- Unit Converter
Reporting an error
If you notice an incorrect result, formula or translation, send us the calculator name, your inputs, the result you got and the result you expected. info@art-visum.hr · About the project