Join them with a flexible transition coupling sized to the outside of both pipes and tightened with stainless steel clamps. The IPC and IRC call for a mechanical joint like this between two kinds of drain pipe.
Rigid white plastic, glued joints
Heavy metal pipe, hub or no-hub
What fits
A flexible transition coupling sized to both pipes, standard or shielded
On the package: Cast iron x plastic coupling, e.g. Fernco 1056-44 or Proflex 3000-44 for 4 in.
the codes call for a mechanical joint to ASTM C1173, C1460 or C1461 between different drain materials
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Keep water out of the drain while it’s open: don’t run the fixtures that empty into it.
Size it by outside diameter: Fernco’s couplings seal on the pipes’ outsides. Its 1056-44 takes 4.02" to 4.65" on both ends, which covers 4" PVC (4.500") and 4" cast iron (4.30" to 4.38").
Standard or shielded: Fernco rates its standard flexible couplings for use above and below ground, and makes its Proflex couplings, with a stainless steel shield, for above-ground transitions. Proflex 3000-44 joins 4" cast iron to 4" plastic.
Tighten the clamps to the maker’s torque (60 inch-pounds on Fernco’s) and test the joint before it’s buried or closed in.
4" PVC is wider than 4" cast iron, and the codes join plastic to other drain pipe with an approved adapter fitting, so a coupling made for both pipes sits between them. The IPC and IRC join two drain materials with a mechanical joint with a rubber seal, approved for the use and installed to the maker’s instructions. Your local code decides which coupling a joint needs.
Measure both pipes’ outsides before you choose the coupling: see 2" pipe OD and 4" pipe OD for each kind.
What the code says
Different drain materials (IPC 705.16, IRC P3003.13): a mechanical joint conforming to ASTM C1173, ASTM C1460 or ASTM C1461, with a rubber (elastomeric) seal, approved for the application and installed to the maker’s instructions.
Plastic to other pipe (IPC 705.16.4, IRC P3003.13.4): an approved adapter fitting. Plastic to a cast iron hub takes a caulked joint or a mechanical compression joint.
Editions: the IPC 2018 and 2021 describe the joint as “of the compression or mechanical-sealing type”. The 2024 edition drops those words and keeps the same three ASTM standards.
Your state or city adopts one of these codes, sometimes with changes, and its building department decides what applies to your house.
Standard or shielded coupling
Fernco’s standard couplings are a flexible sleeve tightened with stainless steel clamps, made to ASTM C1173. Its Proflex couplings add a stainless steel shield around the sleeve and are made to ASTM C1460.
Hub or no-hub cast iron
No-hub cast iron has plain ends, joined by banded couplings made to CISPI 310, ASTM C1277 or ASTM C1540. Hub and spigot pipe has a wider bell on one end. For cast iron pipe in a cast iron hub, the IPC’s compression gaskets conform to ASTM C564 and are squeezed tight as the pipe goes fully in. Where plastic meets a cast iron hub, the code calls for a caulked joint or a mechanical compression joint, so check with your building department before you fit plastic into an old hub.
These couplings are for drain, waste and vent pipe. Never use one on gas pipe or a water supply line. If the cast iron is cracked or the work is inside a wall or under a slab, call a licensed plumber.
Not under the IPC and IRC. They join plastic pipe to other materials with an approved adapter fitting, and the only glued joint between two materials they allow is ABS to PVC, at the start of the building sewer.
Can one coupling join 4 inch cast iron to 3 inch PVC?
Yes, a reducing coupling. Fernco's 1056-43 takes 4.00 to 4.65 inches across on one end and 3.00 to 3.62 inches on the other.
What size Fernco fits 3 inch cast iron to 3 inch PVC?
Fernco's Proflex 3000-33 joins 3 inch cast iron to 3 inch plastic. Its standard 1056-33 takes 2.95 to 3.62 inches across on both ends.
The code text is from the IPC and IRC as adopted by the states named in the list, the couplings from Fernco’s and Oatey’s own product pages, and the pipe sizes from Charlotte Pipe’s submittals and technical manual. Checked against published data from Charlotte Pipe, International Code Council, Fernco and Oatey.
See all 8 sources
Charlotte Pipe, Plastics Technical Manual (2026), product data: average outside diameter and minimum wall of PVC Schedule 40 and 80, PVC DWV, PVC and ABS foam core, 4" sewer and drain pipe, and CTS CPVC
Charlotte Pipe, Cast Iron No-Hub Submittal Package (2026), Table 1: barrel and spigot outside diameters (CISPI 301, ASTM A888); shielded hubless couplings
Charlotte Pipe, Cast Iron Service Weight Submittal Package (2025): hub and spigot pipe barrel outside and inside diameters (ASTM A74)
International Plumbing Code 2018 (as adopted in Georgia and Oklahoma), 2021 and 2024 (as adopted in South Carolina), section 202 (building drain, building sewer) and sections 705.2, 705.3.2, 705.3.3, 705.10, 705.16 and 705.16.4: ABS, cast iron and PVC joints, joints between different materials, plastic to other piping, the one ABS to PVC solvent-cement joint (ASTM D3138)
International Residential Code 2021, sections P3003.13 and P3003.13.4: joints between different drain materials, plastic to other piping, the one ABS to PVC solvent-cement joint (as adopted in South Carolina)
Fernco, Standard Flexible Couplings page: series by pipe pair (1002 clay, 1056 cast iron, plastic or steel), outside diameter ranges per part, above and below ground, Strong Back RC for buried joints, 60 in-lb torque, ASTM C1173, not for pressurized water
Fernco, Proflex shielded couplings page: above-ground DWV transitions, stainless steel shield, ASTM C1460, 3000 series cast iron to plastic and 3005 series plastic to plastic
Oatey, ABS to PVC Transition Medium Bodied Green Cement product page: DWV-sewer, non-pressure transition joints up to 6", ASTM D3138
General guidance from manufacturer and standards data. Parts vary by brand, so check the size printed on the package against what you measured. For gas lines or anything inside a wall, use a licensed plumber.