Asphalt Price Index: How State DOT Binder Indexes Work

An asphalt price index is a monthly price for liquid asphalt binder that a state transportation department publishes and uses to adjust paving contracts. Binder is the oil-based glue that holds hot mix together, and its price can swing between the day a contractor bids a job and the day the mix goes down. The index lets the agency and the contractor share that risk instead of everyone padding their bids. Here is how the index is built, how adjustment clauses use it, and what it does and does not tell you about the price of a ton of asphalt.

Asphalt cost per ton →

The short answer

What it prices: liquid binder alone, usually a common grade such as PG 64-22, per ton of binder.
Who publishes it: state DOTs. FHWA's contract administration pages link examples from Caltrans, New York State DOT and Oklahoma DOT, and many other states post their own.
How it is used: a contract locks in a base index. Each month the current index is compared with the base, and the contractor is paid more or less for the binder in the mix placed.
What it is not: a hot-mix price. Binder is a small share of a ton of mix by weight, so the mix price moves much less than the index.

How a state builds the monthly index

Each agency writes its own method, but they follow a similar pattern. North Carolina's published reference is a clear example:

Binder suppliers on the department's terminal list report what they charge for PG 64-22 at each terminal, F.O.B., as of the 1st of the month.
The department drops the single highest and single lowest price and averages the rest, rounded to the nearest cent. That average is the monthly price index.
The index is posted early in the month, by the fifth day in North Carolina's case.
If fewer than four terminals report in a month, the adjustment for that month is not made.

South Carolina describes its index as an average of quotations from current binder suppliers, effective on the first of each month, and posts it in a spreadsheet. Other states use their own supplier surveys or a published market report. The details matter if you bid public work, so read the special provision in the actual contract.

Base index vs monthly index

The base index is the value frozen into a contract when it is bid. North Carolina uses the monthly index from two months before the letting; a contract let in April would use February's index. That base stays fixed for the life of the contract.

The monthly index is the value for the month when work is placed or paid. The difference between the two drives the adjustment. If the monthly index is higher, the contractor gets an extra payment; if it is lower, the agency deducts.

How a price adjustment clause works

Clauses differ, but most include these pieces:

Eligible items. Usually plant mix, and sometimes surface treatments, microsurfacing or patching. South Carolina publishes binder content factors, the tons of binder per unit of work, for each eligible item.
Binder quantity. The agency figures the tons of binder in the mix placed, typically from the approved job mix formula or a fixed factor. North Carolina applies the adjusted price to the theoretical binder quantity and treats binder coming from recycled material differently.
Trigger. Some states adjust on any change, others only past a threshold. South Carolina applies adjustments when the index varies more than 5 percent from the base.
Limits. Many clauses cap the index once the contract completion date passes, so a late job does not collect on later price spikes. Georgia's guidelines also exclude projects with less than 366 calendar days from letting to the specified completion date.

A worked example with made-up numbers

The numbers below are invented to show the math. They are not current prices.

Suppose the contract's base index is $600 per ton of binder and this month's index is $660, a $60 increase.
Suppose the mix contains 5.5 percent binder by weight, so each ton of mix holds 0.055 tons of binder.
The change per ton of mix is 0.055 × $60, or $3.30.
If the contractor placed 1,000 tons of mix that month, the adjustment is 1,000 × $3.30, or $3,300 added to the payment.

Notice how a 10 percent jump in the binder index turned into a few dollars per ton of mix. That is why the index makes headlines in the paving trade but does not translate one to one into the price of a driveway.

Why the index matters even if you never bid public work

It explains price moves. Plants buy the same binder that state indexes track. When the index climbs for several months, expect plant prices per ton to follow. Our guide to how much asphalt costs per ton covers the other pieces of a delivered price.
It shows up in private contracts. Some commercial paving agreements borrow a state index for an escalation clause so neither side has to guess on a long job.
It hints at timing. Prices often shift with the season, and many plants shut down or slow over winter; see why asphalt plants close in winter.

How to find your state's asphalt price index

Search your state DOT's construction or contracts pages for "asphalt binder price index," "asphalt cement index" or "bituminous price adjustment." Look for three things: the binder grade the index tracks, the date each month's value takes effect, and the special provision that explains how adjustments are calculated. If you are reading a quote rather than a contract, our explainer on what PG 64-22 means on an asphalt quote helps decode the binder grade.

Questions people ask

What is an asphalt price index?

It is a monthly price for liquid asphalt binder, usually a grade such as PG 64-22, published by a state transportation department. Agencies use it to adjust paving contract payments when binder prices rise or fall after bids are opened.

Is the asphalt price index the price of hot mix per ton?

No. The index is the price of the liquid binder alone, per ton of binder. Binder is only a small share of a ton of hot mix by weight, so a change in the index moves the mix price far less than the headline number suggests.

How is the monthly index calculated?

It varies by state. North Carolina, for example, averages the terminal F.O.B. selling prices that suppliers report for PG 64-22 on the first of the month after dropping the single highest and lowest prices.

What is the base index in a paving contract?

It is the index value locked in for the contract at bidding time. North Carolina uses the monthly index from two months before the letting. Later monthly values are compared against that base to figure the adjustment.

Do all projects get asphalt price adjustments?

No. Many agencies limit the clause to certain contracts or items. Georgia, for example, makes no adjustment on projects with less than 366 calendar days between letting and the specified completion date, and South Carolina adjusts only when the index moves more than 5 percent.

Can homeowners use the asphalt price index?

Mostly as a signal. Residential driveway quotes are rarely tied to an index, but a rising state index can explain why plant and contractor prices moved. Some private contracts include an escalation clause that points to a state index.

Related guides

See asphalt cost per ton →

As an Amazon Associate, AsphaltTons.com earns from qualifying purchases. Product links below are sponsored.