Tools · Estimating
A build cost estimate
built from your rate, not ours.
Enter your own areas and your own rate per square metre, add the site costs, allowances, contingency and margin the job carries, and the calculator works up to a price and back down to the all-in rate your figures imply. It ships no benchmark rates, on purpose, and there is a tool below it for deriving your rate from a job you have already built.
01 / The calculator
Work it up
Every field starts empty. One area with its rate is enough to begin, and each figure you add appears as its own line in the result.
What your figures produce
Enter at least one area with its rate. Everything else is optional, and every line you add appears here.
Where your rate comes from
The rate that predicts your next job is the one your last job produced. Take a completed build, use its actual construction cost excluding GST and excluding your margin, and divide it by the area you measured. That figure carries your trades, your specification and your region, which no published average can.
Actual cost to build, excluding GST and excluding margin. Site costs excluded if you want a clean construction rate.
Measured on the same basis you will use above, so the rate and the area agree.
Your rate
Enter a completed job cost and the area it covered.
02 / The rate
Why there is no rate in this calculator
Almost every build cost calculator on the internet pre-fills a dollar figure per square metre. It is the feature that makes them feel useful and the reason their output cannot be trusted. A rate carries an entire set of assumptions inside a single number, the region, the site, the specification, the trade base, the overhead structure and the year, and none of those assumptions travel with it when it is copied onto a page.
The consequence is not a small error. Two builders pricing identical drawings honestly arrive at genuinely different rates, because their businesses are different, and a single published figure has to be wrong for at least one of them and usually for both. A feasibility built on a borrowed rate looks precise and is not, which is more dangerous than an obvious guess, because a decision gets made on it.
So this page supplies the arithmetic and asks you for the number. The rate finder below the calculator derives it the only way that is defensible, from a job you have already built. Take a completed build, use its actual construction cost excluding GST and excluding margin, divide by the area you measured, and repeat across a few comparable builds. What comes out carries your trades and your region, which no average can. The reasoning behind rates in general, and what a headline rate quietly leaves out, is in the cost per square metre reference.
03 / Worked example
Following the arithmetic through
The rates below are round placeholder numbers chosen so the arithmetic is easy to follow. They are not benchmarks, they are not a market estimate, and they are not a suggestion. Replace every one of them with your own. The point of the example is the sequence, not the figures.
- Living area 200 m² at a placeholder $1,000 per m², which is $200,000. Garage 40 m² at a placeholder $500 per m², which is $20,000. Construction from the rates, $220,000.
- Site costs and external works, $50,000. These are priced from the site, never from a rate, because they have almost no relationship to floor area.
- Allowances and provisional sums carried in the price, $30,000. Cost before contingency, $300,000.
- Contingency at 5%, which is $15,000. Total cost to build, $315,000.
- Margin at 20%, applied as a margin rather than a markup, so the price is $315,000 divided by 0.8, which is $393,750. The margin itself is $78,750.
- GST at the rate entered, 10% in this example, is $39,375, giving $433,125 including GST.
- The implied all-in rate is $393,750 across 240 m² of priced area, which is $1,640.63 per m². Notice how far that sits above the $1,000 the example started from. That gap is the entire reason headline rates mislead.
That last line is the most useful output on the page. It is the number a client would quote back at you, and it exists nowhere in the inputs. Anyone comparing a rate they read somewhere against a builder's all-in figure is comparing two different things, which is the trap the build cost feasibility guide unpacks layer by layer.
04 / Common mistakes
Five ways a square metre estimate misleads
- Mismatching the rate and the area. A rate derived on total area under roof, applied to living area alone, understates the cost by whatever the garage and alfresco were worth. Derive and apply on the same basis, every time.
- Assuming site costs scale with floor area. They do not. Excavation, retaining, service connections and access are functions of the site, and they are the single largest source of variance between two builds of identical size.
- Carrying an old rate forward. A rate from a job priced two years ago prices that job, not this one. Rebuild it from recent completions rather than indexing it in your head.
- Entering a markup where the field says margin. The two produce different prices from the same cost. Convert first with the margin and markup calculator if your habit is to think in markup.
- Treating the output as a price. It is a feasibility check. The number you contract on comes from a takeoff measured against the drawings and priced against a maintained cost database, which is a different exercise with a different tolerance.
05 / FAQ
Common questions.
Properly, from a takeoff: measure the quantities off the drawings, price them against a maintained cost database, add the preliminaries, the allowances and a contingency, then apply your margin. A rate per square metre is the faster method, and it is a different thing, an orientation check rather than a price. You multiply an area by a rate you can defend, add everything the rate does not cover (site costs, external works, allowances), add contingency, then add margin. That is the sequence this calculator follows. It is enough to test whether a project is worth pricing properly. It is not enough to sign a contract on.
Because any rate we supplied would be wrong for most people who used it, and confidently wrong is worse than absent. A published rate cannot know your region, your site, your specification level, your trade base, your overhead recovery or what the last twelve months did to your prices. Two builders pricing the same drawings honestly land on genuinely different rates, and neither is making an error. Every calculator that pre-fills a rate is either guessing on your behalf or averaging a market you may not be in. This one asks you for the number, and the tool underneath the calculator helps you derive it from a job you have already built.
From your own completed jobs, which is what the rate finder below the calculator does. Take a build you finished, use its actual construction cost excluding GST and excluding your margin, and divide by the area measured on the same basis you will use in the calculator. Do that for three or four comparable builds and you have a range that carries your trades, your specification and your region. If you have no comparable history, a quantity surveyor can produce a rate for the specific project, and your cost database can build one from first principles. What none of those sources is, is a number off the internet.
Whichever basis your rate was derived on, and the two have to match or the answer is meaningless. Some builders quote a rate against enclosed living area only, others against total area under roof including garage and alfresco. A rate derived on total area and applied to living area alone will overstate the cost badly. This calculator separates living area, garage and covered outdoor space so each can carry its own rate, which is the more honest structure, because a garage and an alfresco genuinely do not cost what a kitchen costs. The measurement question is covered further in the cost per square metre reference.
As a margin, meaning a share of the finished price. A total cost of $315,000 with a 20% margin produces a price of $393,750, because 20% of $393,750 is the $78,750 of gross profit. A 20% markup on the same cost would produce $378,000 and return only a 16.67% margin. The difference on one build is substantial, and it is the single most common arithmetic error in residential pricing. If the percentage you are used to entering is a markup, convert it first with the margin and markup calculator.
Your own figure, and it should be a decision about identified risk rather than a habit. Contingency covers the things you know can happen on this job but cannot yet price: the ground conditions before the geotechnical report lands, the extent of rectification in a renovation once linings come off, the exposure in a long programme. A greenfield slab on a flat, known site does not carry the same risk as a hillside knock-down rebuild in an established suburb, so a single standing percentage across every job is either too much on the easy ones or not enough on the hard ones. The estimating contingency reference sets out how to size and track it.
It inherits every assumption in the rate, which is the honest answer, and it is why the output should be treated as a range rather than a figure. The two things that move it most are the specification level and the site. A rate derived from a project home will not price a custom build with the same finishes, and a rate derived from a flat lot will not price a sloping one, because site costs are not a function of floor area at all. Use the result to decide whether a project is worth pricing properly, and use a takeoff against a cost database to produce the number you actually contract on.
You can use it to understand the shape of a quote, but not to judge whether the price is fair, because you would have to supply the rate and the rate is the whole question. What it is genuinely useful for is seeing where a price comes from: how much of it is construction, how much is site and external works, how much is allowances that will move when selections are made, and how much is contingency and margin. That structure is what makes two quotes comparable, far more than the headline number. A quote that cannot be broken down this way is the finding, rather than the price it arrives at.
Both figures are shown. The price excluding GST is the contract value, and the price including GST is what a residential client pays, calculated at the rate you enter rather than a rate this page assumes. Every input is treated as GST-exclusive, which is the only way the arithmetic holds together, because your cost inputs are amounts on which you claim credits. How GST applies to your contracts, and when it is attributable, is a question for your accountant against current ATO guidance.
06 / Keep reading
The references behind this calculator
The knowledge nodes on rates, cost data and contingency, the feasibility guide, the companion calculators, and the estimating that replaces the guess.
Then price the real job from the drawings.
A rate estimate tells you whether a project is worth pricing. VIABUILD does the pricing, plans in, quantities measured, your own rates and assemblies applied, and the estimate carried straight into the budget the job is then run against.