How we calculate

The formula

cost = Σ over line items of:
         quantity × ( material + equipment + hours × hourly rate )

hourly rate = national median wage        ← BLS OEWS, by occupation
            × area multiplier             ← BLS OEWS, 530 areas
            × benefit load                ← BLS ECEC
            × contractor markup           ← derived from NAHB, estimated

Quantities come from published code where code sets them. For septic systems that is design flow, tank capacity and soil loading rate, all of which are regulated numbers rather than opinions.

Why only labour moves with your ZIP code

The area multiplier is the median hourly wage for that occupation in your metropolitan area, divided by the national median for the same occupation. A plumber’s median is $30.67 nationally and an equipment operator’s is $28.78 (May 2025); the multiplier moves those up or down for where you live.

It is applied to the hours only, never to materials. A precast concrete tank costs much the same in Ohio as in Nevada; what genuinely differs between markets is what the crew installing it charges. Applying a metro multiplier to the whole total would inflate the price of concrete for living in an expensive city, which is exactly the mistake that makes these tools worthless.

Where the BLS does not publish a given occupation for a given area, we fall back to the broader occupational group, and if that is missing too, to the national figure. We never borrow the value of a neighbouring area.

From a wage to a bill

The BLS publishes what workers are paid, not what contractors charge. Two factors close that gap, and they are not equally solid.

The benefit load is measured and published. The markup is not: no public source reports margins by trade for US residential work. We derive a range from NAHB’s published remodeler gross margin and say plainly that applying a general remodeler’s margin to a septic installer is our assumption.

Note the conversion, because it trips people up: a gross margin of 30% on revenue is a 43% markup on direct cost, not 30%.

Equipment

Machine time comes from the FEMA Schedule of Equipment Rates, a federal published rate covering depreciation, overhead, maintenance, field repairs, fuel, lubricants and tyres. FEMA states explicitly that the operator is not included, which is why we add operator hours separately from wage data. Had we missed that, every excavation on this site would be overstated by roughly 40%.

Material prices are estimates, and here is why

There is no public, machine-readable source of residential construction material prices in the United States. We looked properly before concluding that. National retailers block automated access outright; specialist suppliers serve their prices through JavaScript rather than in the page; state highway bid tabulations are internally inconsistent, listing a 15-inch pipe at a third the price of a 4-inch one; and searching for official component-level cost data returns nothing but other cost-guide sites, none of which cite a source either.

That last point is worth sitting with. Every competitor publishes vague unsourced ranges not out of laziness but because they cannot source them either. The difference here is that we say which figures are estimated.

So material prices are modelled: reasoned from what the item is, how much of its price is freight, and what makes it vary between markets. Each line carries its own uncertainty band, shown on the line, and the reasoning is written into the page. Bulk aggregate is wide because haulage dominates; PVC pipe is narrow because it is a light, standardised commodity; precast concrete is the widest because it is heavy and made locally.

We track the BLS Producer Price Index for the relevant commodity groups so we know when an estimate has drifted and needs revisiting, rather than reviewing everything blindly once a year.

Where the quantities come from

Septic system sizing is set by state and often county rules; there is no federal standard. We model it on Minnesota Rules 7080, because that code publishes all three pieces we need — design flow, tank capacity and soil loading rate — in one consistent body of rules, and its values sit in the usual range for other states.

Your local health department has the final say and its requirements may differ. Treat the sizing here as a well-founded starting point for checking a quote, not as a permit application.

What is deliberately left out

Municipal and county permit fees are excluded from every estimate on this site. They vary enormously by jurisdiction and there is no national source, so leaving them out and saying so is more honest than inventing an average. Each calculator lists its own exclusions on the page.

These figures are orientation for checking a quote. They do not replace a written estimate from a licensed contractor who has seen your site.

If you think a number is wrong

Every estimated line states its reasoning precisely so that someone who does this work for a living can point at the specific assumption that is off. That is the point of writing it down. A figure with no reasoning behind it cannot be corrected; one with reasoning can.