Strip Foundation Calculation
About Strip Foundation Calculation
The strip foundation calculator determines the foundation geometry, concrete volume, footing area, outer side surface area, and, if needed, the estimated quantities of formwork and reinforcement materials. This type of calculation is used for a preliminary estimate of concrete, timber, and reinforcement consumption when planning a monolithic strip foundation for external walls and internal load-bearing lines.
The calculation is suitable for private construction and for preparing an initial cost estimate. It is based on a geometric evaluation of the strip lengths according to the selected plan layout, after which the concrete volume, bearing area, formwork parameters, and reinforcement lengths are determined step by step from the resulting total length.
Guidelines and recommendations
Geometric calculation scheme
Main strip length. First, the calculator determines the total length of the outer contour from dimensions A and B in mm. For a rectangular contour, the expression Lout=2×(A+B)-4×t is used, where t is the width of the main strip. The subtraction of 4×t accounts for the fact that the length is taken along the axial geometry of the assembled strip rather than along the outer overall dimensions of the rectangle.
Internal strips. If the selected layout includes internal sections, their length is determined separately according to the plan shape. In some layouts, the internal length depends on the offset AB, while in others it depends on the clear internal size B-2×t or A-2×t. The calculator then adds the lengths of the main and internal strips and obtains the total length Ltotal in m.
Selection of internal strip width. A separate width in mm is used for internal walls. If the load-bearing wall C option is enabled, one of the internal strips is taken with the same width as the main strip, which affects the total main length and all subsequent calculations for concrete and reinforcement.
Concrete volume and areas
Concrete volume. For each strip group, the volume is calculated as the product of length, width, and height. The calculation logic is expressed by the formulas Vout=Lout×tout×H and Vin=Lin×tin×H with conversion from mm³ to m³. The total volume is Vtotal=Vout+Vin.
Footing area. The bearing area is calculated separately for the main and internal strips as the sum of rectangular bands. The expression Afoot=Lout×tout+Lin×tin is used, where lengths and widths are converted to m. This result helps estimate the total area transferring load to the soil.
Outer side surface. The side surface area is calculated only for the outer perimeter, without internal walls. The formula is Aside=Pout×H, where Pout=2×(A+B) in m and H is the strip height in m.
Formwork calculation
Net formwork area. If formwork calculation is enabled, the calculator first determines the area of the two side faces of all strips. The expression Afw=2×Ltotal×Hfw is used, where Hfw is the formwork height in m.
Number of board rows. The calculator then divides the formwork height by the working board width and always rounds the result up. In other words, the number of rows is ceil(Hfw/b), where b is the board width in mm. This means that even an incomplete final row is counted as a full board row.
Total board length, area, and volume. The total board length is determined as 2×Ltotal×n, where n is the number of rows. The board area is then calculated as this length multiplied by the board width, and the board volume as the area multiplied by the board thickness. The number of boards is obtained by dividing the total length by the length of one board and rounding up.
Longitudinal reinforcement calculation
Lap length of longitudinal bars. For bar splicing along the length, the calculator uses the fixed rule llap=40×Ø, where Ø is the diameter of the longitudinal reinforcement in mm. For example, for a diameter of 12 mm, the lap length is taken as 480 mm.
Number of bars along the length. The calculator first determines whether one bar of the specified length is sufficient for the entire line. If not, each additional bar contributes not its full length, but its length minus the lap. Therefore, the final number of bars for one longitudinal line is chosen so that the entire length Ltotal is covered while accounting for repeated lap splices.
Total length of longitudinal reinforcement. After determining the number of bars for one line, the calculator multiplies the result by the number of longitudinal bars in the strip cross-section. This gives the total length of longitudinal reinforcement, with all lap lengths already included.
Stirrup calculation
Internal stirrup size. The stirrup size is not based on the full strip width and height, but on the internal outline of the reinforcement cage. For this purpose, 2×c is subtracted from both the strip width and the strip height, where c is the concrete cover in mm. This gives the dimensions t-2×c and H-2×c.
Stirrup lap length. For the stirrup, the calculator takes the lap length equal to the internal strip width. Therefore, the length of one stirrup is calculated as lst=2×((t-2×c)+(H-2×c))+(t-2×c). If the internal strips have a different width, the stirrup length for them is calculated separately.
Number of stirrups. The number of stirrups along each strip group is determined from the spacing, with rounding up and adding one end element. The logic is N=ceil(L/step)+1, where L is the length of the corresponding strip in mm and step is the stirrup spacing in mm.
Bars required for stirrup fabrication. After calculating the total number of stirrups, the calculator determines how many straight bars of the specified length are needed for bending them. It also accounts for the fact that stirrups for the main strip and internal walls may have different sizes, and that the remaining length of one bar may be used for another stirrup type.
Practical guidelines
Concrete cover. In preliminary strip foundation calculations, a value of about 40 mm is often used, and this is the default value in the calculator. Increasing the concrete cover reduces the internal stirrup size and slightly lowers the reinforcement length required for one stirrup.
Bar lengths. Common bar supply lengths used in calculations are 6000 mm and 12000 mm. The longer the bar, the fewer lap splices are needed and the lower the total consumption of longitudinal reinforcement will be.
Stirrup spacing. For preliminary estimates, spacing in the range of about 200-500 mm is often used depending on the cross-section and the chosen reinforcement arrangement. Reducing the spacing directly increases the number of stirrups, the total length of transverse reinforcement, and the number of bars required for fabrication.
Related European standards. In terms of engineering context, strip foundations are related to EN 1992-1-1 Eurocode 2. Design of concrete structures. Part 1-1: General rules and rules for buildings, EN 1997-1 Eurocode 7. Geotechnical design. Part 1: General rules, EN 1997-2 Eurocode 7. Geotechnical design. Part 2: Ground investigation and testing, EN 206 Concrete. Specification, performance, production and conformity, and EN 13670 Execution of concrete structures. In this calculator, they serve as the normative background for selecting concrete cover, reinforcement principles, concrete requirements, and foundation soil checks, although the calculation itself is geometric and quantity-based.
FAQs
Why is the concrete volume calculated separately for the main strip and internal walls?
This is because these foundation parts may have different widths. The calculator first determines the length of each strip group, then multiplies it by the corresponding width and height, and finally sums the volumes into one total concrete result.
Why is the side surface area not equal to the area of all side faces of the foundation?
In this calculation, the displayed value is specifically the outer side surface area along the external perimeter. Internal walls are not included in this figure, which makes it useful for a preliminary estimate of external waterproofing or finishing of the visible part of the strip.
Why are the quantities of boards and bars rounded up?
This is because construction estimates are based on whole items rather than fractions of an item. The calculator deliberately selects the next higher whole number so that the material quantity is sufficient for the work without shortage.
How does the reinforcement bar length affect the result?
It affects the number of splices and the total reinforcement consumption. If one bar is shorter than the full strip length, the calculator adds lap lengths of 40×Ø, so shorter bars increase the total length of longitudinal reinforcement.
Can this calculation be used to size the foundation according to soil conditions and building load?
This calculator is well suited for estimating concrete, reinforcement, and formwork quantities, but the selection of strip width and height should be confirmed by a separate engineering check. That verification usually follows Eurocode 7 and uses soil data, building loads, settlement assessment, and bearing capacity calculations.