How to use this calculator
- Enter actual ODs. Use actual outside diameters, not nominal pipe size.
- Enter reducer length. Use the fitting, shop drawing or proposed layout length between end planes.
- Choose reducer type. Use concentric for aligned centerlines, flat-side eccentric for one flush side, or custom offset for a special layout.
- Check side lengths. Compare centerline offset, long/short side length, taper angle and shell-area estimate before moving to a shop pattern.
How it works
Let R be the large-end radius, r be the
small-end radius, DeltaR = R - r, L be the
reducer axial length and e be the centerline offset.
A concentric reducer uses e = 0. A one-side-flat eccentric
reducer uses:
e = (OD1 - OD2) / 2
The steep and flat side profile lengths are:
L_long = sqrt(L^2 + (DeltaR + e)^2)
L_short = sqrt(L^2 + |DeltaR - e|^2)
Taper angles come from the same right-triangle geometry:
theta_max = atan((DeltaR + e) / L)
theta_min = atan(|DeltaR - e| / L)
Use the pipe saddle cut calculator for branch coping, the pipe miter cut calculator for fabricated elbows, the pipe rolling offset calculator for offset spool travel and the cone flat pattern calculator for true concentric cone/frustum development.
Worked example
Verified against the live calculator
A flat-side eccentric reducer from 6.625 in OD to 4.5 in OD has a centerline offset of 1.0625 in. With an 8 in reducer length, the long side is about 8.277 in, the short side is 8.000 in and the maximum side taper is about 14.9 degrees.
Frequently asked questions
How much offset does an eccentric reducer need to keep one side flat?
Use half the diameter difference: e = (OD1 - OD2) / 2. This shifts the small-end centerline enough that one outside side stays flush with the large end.
Is this a concentric reducer length chart?
No. It uses the length you enter and calculates the resulting reducer geometry. Standard reducer lengths depend on fitting standard, material, size series and manufacturer.
Does this make an eccentric reducer flat pattern?
No. It gives side lengths, taper angles, circumferences and a shell-area estimate. Production eccentric reducer flat patterns need triangulation, CAD or shop layout allowance for thickness, seams and forming.
What is the difference between concentric and eccentric reducers?
A concentric reducer keeps both pipe centerlines aligned. An eccentric reducer shifts the small end centerline; a common flat-side eccentric reducer shifts it by half the OD difference.
Method & assumptions
- Uses actual outside diameters and ideal circular end planes.
- Concentric mode sets centerline offset to zero; flat-side eccentric mode sets offset to half the OD difference.
- Side lengths are profile true lengths, not fitting standard lengths.
- Shell area is a numerical ruled-surface estimate for material takeoff; it is not a production flat pattern.
- Does not include ASME fitting dimensions, manufacturer reducer charts, wall thinning, end prep, weld gap, bevel, code acceptance, flow loss, triangulated eccentric flat patterns, forming stretch or shop fabrication allowances.