Formula fields

A formula derives a quantity from other quantities. It is what turns a bill from a list of numbers into something that responds to a change.

Measure the slab once. Let the mesh, the membrane, the sand bed and the concrete all derive from it. Change the slab and all four follow.

Variables

A formula refers to formula variables — named values you define on the costing. Add Formula Variable creates one, and Assembly Values groups the ones belonging to an assembly.

A variable is a value with a name: SlabArea, WallHeight, ExternalPerimeter, RoofPitch. Set it once and use it in as many formulas as you like.

Take-off fields can supply variables too, so a measured value becomes available to every formula in the costing.

Writing a formula

Formulas use ordinary arithmetic: +, −, ×, ÷, brackets, and the variables you have defined.

Item	Formula	Reads as
Slab mesh	SlabArea * 1.05	Slab area plus 5% laps
Vapour barrier	SlabArea * 1.1	Slab area plus laps and turn-ups
Slab concrete	SlabArea * SlabDepth	Area × depth
Edge formwork	SlabPerimeter	Straight from the perimeter
Wall paint	WallLength * WallHeight * 2	Both faces
Skirting	WallLength - DoorOpenings	Wall run less the doors

Worked example: a slab that looks after itself

Set up two variables on the costing:

Variable	Source	Value
SlabArea	Take-off field Slab area	186.40
SlabDepth	Typed	0.10

Then price five items off them:

Item	Unit	Formula	Quantity
Concrete N32 supply and place	m³	SlabArea * SlabDepth	18.64
SL72 mesh	m²	SlabArea * 1.05	195.72
Vapour barrier	m²	SlabArea * 1.1	205.04
Bedding sand	m³	SlabArea * 0.05	9.32
Concrete pump	item	— (typed 1)	1

Now the client adds a metre to the alfresco. You drag one corner in Take-Off, the field goes to 194.80, and four of those five quantities update themselves. The pump stays at one, correctly, because it was typed.

That is the entire argument for formulas in one example.

Where formulas take priority

A field that gets its quantity from a formula is read-only in Take-Off. The formula is the source of truth, so Take-Off shows it and refuses to measure into it.

If you want to measure the field off a drawing instead, clear the formula first.

When not to use a formula

• When the relationship is not real. If the mesh area only usually follows the slab, a formula will be silently wrong on the job where it does not.
• For one-offs. A formula for a single item you will never revisit is more work than typing the number.
• When the working is the point. If you want the derivation visible as a set of dimensions, use the calc sheet instead.

Checking your formulas

Two habits worth having:

1. Sanity-check the first result. A formula that produces 1,864 m³ of concrete instead of 18.64 has a decimal in the wrong place, and it will look plausible in a total.
2. Change a variable and watch. Nudge SlabArea and confirm every dependent item moves. If one does not, it was typed rather than derived.

Name variables so the formula reads like a sentence

SlabArea * SlabDepth is obviously right. A1 * D is not, and you will not remember what D was. The name costs nothing and is the difference between a bill someone else can check and one only you can.

Keeping formula results current

A formula’s answer is worked out once and stored, so it has to be recalculated when something it refers to changes. The BOQ now does that for you.

Change a quantity and, once you stop typing for a moment, the grid quietly recalculates every formula in the costing. You will see Updating formulas… appear briefly next to the toolbar’s circular-arrows button. There is nothing to click and no message when it finishes — the figures are simply right.

It waits for a pause rather than recalculating on every keystroke, so keying quantities down a whole trade is one recalculation at the end, not one per row.

What triggers it, and what does not

A formula can only ever refer to three things: a take-off field’s quantity, another item’s quantity, or an item’s productivity rate. So:

Change	Recalculates?
A quantity	Yes
Saving the Calculate Quantity screen	Yes — the method, wastage, size and rounding all change the quantity
Switching an item on or off	Yes — a switched-off item counts as zero in a formula
Changing an item’s rounding	Yes — it changes the quantity the formula reads
Deleting an item	Yes
A cost or a price	No — no formula can read a rate
Adding or importing an item	No — nothing refers to it yet

That is why editing a rate never sets anything off: it genuinely cannot change a formula result. If a costing has no formula-driven items, none of this ever runs.

Refresh Formulas is still there on the toolbar for forcing a recalculation yourself.

A formula it cannot read needs you

If a formula cannot be worked out, the recalculation stops there and a Formula needs attention warning appears on the toolbar, naming the item.

Click the warning and it opens that item’s Calculate Quantity screen on the Formula tab, ready to fix. You do not have to go and find the item yourself — which matters, because a recalculation covers the whole costing and the item it stopped on can be under any cost code, not just the one you are looking at.

Worth clearing rather than living with: every formula item after the broken one is left unrecalculated too. Automatic recalculation pauses in the meantime, otherwise it would keep retrying the same broken formula. Save a working formula and it recalculates and clears the warning by itself.

The same warning appears if you press Save on the Calculate Quantity screen with a formula that cannot be read. The item’s quantity is stored as zero until it is fixed, so the warning is there to stop that zero going unnoticed once the screen is closed.