How do you calculate hydraulic flow requirement for an attachment?

Multiply the attachment's rated flow by a duty-cycle and pressure-loss multiplier. A 28 gpm attachment at a 1.18 multiplier needs an adjusted 33.04 gpm from the carrier.

What is the duty-cycle and pressure-loss multiplier?

It's a factor above 1.0 that accounts for continuous high-duty operation and losses through hoses, couplers, and the return line. 1.10-1.25 is common; heavy continuous work and long hose runs push it higher.

Why is adjusted flow higher than rated flow?

Rated flow assumes ideal plumbing and intermittent use. Real circuits lose effective flow to restriction and run harder on continuous-duty work, so the carrier must be able to deliver more, here 33.04 vs 28 gpm.

What happens if carrier auxiliary flow is below the requirement?

The attachment runs slow, generates excess heat, and can overheat the hydraulic oil. Match continuous auxiliary flow and cooling capacity to the adjusted figure, not the rated one.

Does this tell me the pressure I need?

No. It sizes flow only. Always confirm the carrier holds the attachment's required operating pressure at that flow, since flow and pressure together set actual power.

How does operating time factor in?

Operating time drives the flow-demand-per-hour reference and helps you reason about heat load and cooling over a run, but the adjusted flow requirement itself is set by rated flow and the multiplier.

Construction Machinery & Attachments calculator

Hydraulic Flow Requirement Calculator

Adjusted hydraulic flow requirement is the real-world gallons-per-minute an attachment demands once duty cycle and pressure losses are layered onto its rated flow. Dealers, fleet engineers, and attachment OEMs use it to confirm a hydraulic breaker, mulcher, or auger won't starve or overheat on a given carrier. Rated flow on a spec sheet is a best case; hose runs, fittings, return-line restriction, and continuous high-duty operation all push the practical demand higher. Sizing the carrier's continuous auxiliary flow and cooling to this adjusted number is what keeps the circuit from cavitating or cooking the oil.

What this calculator does

Estimate hydraulic flow demand for construction attachments from tool flow, duty multiplier, and operating time.
screening attachment compatibility with carrier hydraulic flow capacity
It multiplies the attachment's rated hydraulic flow by a duty-cycle and pressure-loss multiplier to give the adjusted flow the carrier must actually supply.

Formula used

Adjusted hydraulic flow requirement = attachment rated hydraulic flow × duty cycle and pressure-loss multiplier
Compare adjusted flow requirement to carrier continuous auxiliary flow and cooling capacity.

Inputs explained

Attachment rated hydraulic flow:
Duty cycle and pressure-loss multiplier:
Operating time:

How to use the result

Use it before pairing an attachment to a carrier, or when troubleshooting an attachment that runs hot or sluggish.
It captures flow only, not pressure; an attachment can meet the flow target and still stall if the carrier can't hold the required psi at that flow.

Current U.S. benchmarks

U.S. housing starts run at 1,177k per year (Census, May 2026), down 8.7% from a year earlier, the demand driver for building products.
Steel mill PPI stands at 348.53 (BLS, May 2026), up 6.7% from a year earlier. New factory orders are up 2.3% year over year (Census).
The U.S. has 21,668 machinery manufacturing establishments employing about 1,086,146 workers (Census County Business Patterns, 2023).

Common questions

How do you calculate hydraulic flow requirement for an attachment? Multiply the attachment's rated flow by a duty-cycle and pressure-loss multiplier. A 28 gpm attachment at a 1.18 multiplier needs an adjusted 33.04 gpm from the carrier.
What is the duty-cycle and pressure-loss multiplier? It's a factor above 1.0 that accounts for continuous high-duty operation and losses through hoses, couplers, and the return line. 1.10-1.25 is common; heavy continuous work and long hose runs push it higher.
Why is adjusted flow higher than rated flow? Rated flow assumes ideal plumbing and intermittent use. Real circuits lose effective flow to restriction and run harder on continuous-duty work, so the carrier must be able to deliver more, here 33.04 vs 28 gpm.
What happens if carrier auxiliary flow is below the requirement? The attachment runs slow, generates excess heat, and can overheat the hydraulic oil. Match continuous auxiliary flow and cooling capacity to the adjusted figure, not the rated one.
Does this tell me the pressure I need? No. It sizes flow only. Always confirm the carrier holds the attachment's required operating pressure at that flow, since flow and pressure together set actual power.

Last reviewed 2026-05-12.