Quick answer
At a glance
A lump sum contract sets one fixed price for a fully defined scope, so the contractor carries the risk of quantity overruns. A unit price contract pays for the actual measured quantity of each work item at a fixed unit rate, so the owner carries quantity risk. Use lump sum when drawings are complete and scope is firm; use unit price when quantities are uncertain (earthwork, utilities, paving, repair work).
Key takeaways
What you need to know
- Lump sum = one fixed price for a defined scope; the contractor absorbs quantity overruns and the owner gets price certainty.
- Unit price = fixed rate per measured unit (per cubic yard, linear foot, ton); final payment follows actual field-measured quantities.
- Choose lump sum when the design is complete and quantities are known; choose unit price when quantities are genuinely uncertain at bid time.
- Many real contracts are hybrids: a lump sum base scope plus unit price items for the uncertain work.
- Watch for unbalanced bidding on unit price jobs, and verify quantities in the field before approving payment.
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How a lump sum contract works
In a lump sum contract, the owner provides complete drawings and specifications, and the contractor commits to delivering that scope for a fixed price. To build the number, the contractor performs a full quantity takeoff, prices labor, material, equipment, and subcontracts, and adds overhead, profit, and contingency.
- Quantity risk sits with the contractor. If the takeoff misses material or conditions are harder than expected, the contractor eats the difference unless it qualifies as a true change in scope.
- Payment is typically tracked through a schedule of values and billed by percent complete (the AIA G702/G703 format is standard on building work).
- Changes to scope are handled through change orders; the base price does not flex with quantities.
Lump sum is the default on most vertical building construction — offices, schools, retail, multifamily — where drawings are complete and quantities are knowable before bid day.
How a unit price contract works
In a unit price contract, the owner publishes a bid schedule listing each work item with an estimated quantity and a unit of measure. Bidders fill in a unit rate for each item; the bid total is the sum of (estimated quantity × unit rate). After award, the contractor is paid for the actual measured quantity of each item at the agreed rate.
- Quantity risk sits with the owner. If the field requires more cubic yards of excavation than estimated, the owner pays for the extra at the bid rate.
- Payment follows field measurement — survey, count, weigh tickets — so documentation and joint measurement matter.
- Changes in quantity adjust automatically within existing bid items; only genuinely new items need a change order.
Unit price dominates heavy civil and public works: roadway and paving, water and sewer utilities, earthwork, dredging, and environmental remediation — anywhere quantities can't be pinned down until the work is underway.
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Key differences at a glance
| Factor | Lump Sum | Unit Price |
|---|---|---|
| Price basis | One fixed total for defined scope | Fixed rate per measured unit |
| Quantity risk | Contractor | Owner |
| Best when | Design complete, scope firm | Quantities uncertain at bid |
| Payment | Schedule of values, % complete | Actual measured quantities × rate |
| Bid comparison | Single number per bidder | Extended totals by line item |
| Change management | Change order for any scope change | Auto-adjusts by quantity; CO for new items |
| Typical use | Building / vertical construction | Heavy civil, utilities, paving, repair |
When to use each
Choose lump sum when:
- Drawings and specifications are complete and the scope is well defined.
- The owner needs price certainty and a fixed budget.
- The work is repeatable or predictable enough to take off accurately.
Choose unit price when:
- Quantities are genuinely unknown — earthwork volumes, linear feet of utilities, tons of asphalt, rock or unsuitable-soil removal.
- The owner wants to pay only for work actually performed.
- The job is a renovation or repair where hidden conditions are likely.
Consider a hybrid when: most of the scope is defined but a few items are uncertain. A common pattern is a lump sum base bid plus unit prices for items like rock excavation, dewatering, or contaminated-soil disposal — combining price certainty with fair payment for the unknowns. Unit prices are also frequently requested alongside bid alternates so owners can adjust scope after bids open.
What it changes for your bid
- On lump sum jobs, your takeoff is your margin. Because you own quantity risk, an incomplete takeoff directly erodes profit — so accurate quantities and a defensible contingency matter. Build markup deliberately rather than guessing; see our guide to calculating construction bid markup.
- On unit price jobs, your rates are your margin. Spread fixed costs and risk into the right line items and verify the owner's estimated quantities against your own takeoff. Be aware of unbalanced bidding — front-loading rates on items expected to overrun. Many public owners scrutinize and can reject materially unbalanced bids.
- Define scope precisely either way. Ambiguity drives disputes; a clear scope of work and disciplined change order management protect both structures.
Payment and documentation
On lump sum work, progress billing tracks the schedule of values — see our construction payment applications guide. On unit price work, every payment ties back to measured quantities, so joint field measurement, survey records, and delivery tickets are the backbone of getting paid accurately and avoiding quantity disputes at closeout.
Bottom line
Lump sum trades price certainty for contractor quantity risk and fits complete, well-defined scopes. Unit price trades final-cost certainty for fair, measured payment and fits uncertain quantities. Match the structure to how well the scope is actually known — and when in doubt on a mixed job, carve the uncertain items out as unit prices inside an otherwise lump sum contract.
Related resources
FAQ
Frequently Asked Questions
What is the main difference between unit price and lump sum contracts?
A lump sum contract fixes a single total price for a clearly defined scope, so the contractor is responsible if real quantities exceed the estimate. A unit price contract sets a fixed rate for each work item and pays for the actual quantity measured in the field, so the final contract value moves up or down with real quantities.
Which contract type is riskier for the contractor?
Lump sum carries more quantity risk for the contractor, because the price is fixed even if quantities run over the estimate. Unit price shifts quantity risk to the owner, since the contractor is paid for whatever quantity is actually installed at the agreed unit rate, though the contractor still owns the risk on its unit rates and productivity.
When do public agencies use unit price contracts?
Public owners and state DOTs commonly use unit price contracts for heavy civil, earthwork, utilities, and paving work where exact quantities are not known until the work is done. Bidders price a published quantity schedule, and payment is based on measured field quantities.
Can a construction contract use both unit price and lump sum?
Yes. Hybrid contracts are common: a lump sum is used for the well-defined scope and unit prices cover the items with uncertain quantities, such as rock excavation, dewatering, or unsuitable-soil removal. This balances price certainty with fair payment for unknown conditions.
How does payment work on a unit price contract?
The contractor and owner measure the actual quantity of each completed work item, then multiply it by the bid unit rate. Progress payments reflect measured quantities to date rather than a percentage of a fixed lump sum, so accurate field measurement and documentation are essential.
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