Get the rough-in right before the vanity goes in, and the faucet and drain line up without a fight.
Rough in the bathroom sink drain 18 to 20 inches above the finished floor, and the hot and cold supply lines 2 to 3 inches higher, around 20 to 22 inches, spaced 4 inches to each side of the sink's centerline. These heights follow standard vanity height, not a fixed plumbing code. What the code does set is the P-trap's water seal (2 to 4 inches under the IPC) and how far that trap can sit from its vent, up to 5 feet for a 1.25 inch trap arm or 6 feet for a 1.5 inch one, per IPC/IRC Table 909.1.
Bathroom sink plumbing comes down to two separate things that get lumped together in most advice: rough-in height, which is a building convention tied to your vanity, and code-governed sizing, which covers trap seals, vent distance, and pipe diameter. Mixing the two up is how a bathroom ends up with a P-trap that won't reach its vent or a supply line buried behind cabinetry.
This guide covers what the labelled plumbing diagram actually shows, the standard rough-in heights for drain and supply lines, the parts list for a typical basin hookup, how P-trap and vent sizing works under US code, and what changes if you're working to UK standards instead.
- Drain stub-out: 18 to 20 inches above the floor. Supply lines: 20 to 22 inches, spaced 4 inches either side of the drain's centerline.
- These heights are trade convention based on standard vanity height (about 32 to 36 inches), not a plumbing code minimum, so a vessel sink or custom vanity can shift them.
- Code-governed numbers, by contrast, are fixed: a P-trap needs a 2 to 4 inch water seal, and its maximum distance from the vent depends on trap size (5 feet for 1.25 inch, 6 feet for 1.5 inch, under IPC/IRC Table 909.1).
- Standard sink rim height is around 31 to 32 inches from the floor; ADA-compliant installations cap the rim or counter at 34 inches maximum (ADA Standards 606.3).
- UK basin plumbing follows Approved Document H instead: minimum 32mm waste pipe and a 75mm trap seal, without the same fixed rough-in height convention used in the US.
Plumbing Diagram Explained
A labelled rough-in diagram is the fastest way to see how the pieces relate before any wall is opened up. Reading one comes down to four reference points: the drain centerline (vertical height and, where it exits the wall, horizontal position), the two supply stub-outs on either side of it, the trap arm connecting the drain to the vent, and the finished sink height once the vanity and basin are installed on top.
Approximate reference only, not to scale. Confirm against your specific sink, vanity, and local code before roughing in.
Two details trip people up on a first read. First, the drain and supply heights are measured to the pipe's centerline, not to its top or bottom edge, so a tape measure reading taken from the wrong point will be off by half an inch or more. Second, the trap arm shown running from the drain to the vent has a maximum allowable length that depends on its diameter, covered in more detail below, so a diagram drawn for a 1.5 inch trap doesn't automatically apply if your fixture uses 1.25 inch pipe.
Rough-In Dimensions and Standard Heights
The heights below are the convention most US plumbers and manufacturers build to for a standard under-mount or drop-in bathroom sink on a vanity. None of them are hard-coded in the plumbing code itself, they follow the standard vanity height of roughly 32 to 36 inches, so treat them as a reliable default rather than a legal requirement.
| Measurement | Inches | Centimeters |
|---|---|---|
| Drain stub-out height | 18 to 20 in | 46 to 51 cm |
| Supply line height | 20 to 22 in | 51 to 56 cm |
| Supply line spacing (each side of centerline) | 4 in | 10 cm |
| Side clearance (centerline to wall) | 15 in min, 20 in recommended | 38 cm min, 51 cm recommended |
| Front clearance | 21 in min, 30 in recommended | 53 cm min, 76 cm recommended |
| Standard sink rim height | 31 to 32 in | 79 to 81 cm |
| ADA maximum rim/counter height | 34 in | 86 cm |
The 34 inch figure isn't a design suggestion, it's a legal maximum for accessible bathrooms under ADA Standards Section 606.3, measured to whichever is higher, the rim or the counter surface. A residential bathroom that doesn't need to meet accessibility code can run a taller vanity, but 31 to 32 inches remains the figure most stock vanities and sinks are built around.
Why the range, not a single number
The 2-inch spread in most of these figures accounts for basin depth. A deep vessel basin sits higher on the vanity, so its drain and supply stub-outs need to land lower in the same range to leave room for the P-trap underneath. Always check the specific sink's spec sheet rather than defaulting to the widest range.
If you're roughing in before the vanity is chosen, working from the lower end of each range (18 inches for the drain, 20 inches for supply) leaves more flexibility, since it's easier to extend a supply line up a couple of inches than to lower a stub-out that's already set in the wall. Our undermount vs. drop-in sink comparison covers how basin depth varies by mounting style if you haven't settled on a sink yet.
Parts You Need
A standard bathroom sink hookup uses a short, predictable parts list. Beyond the faucet and basin themselves, expect to source or confirm the following before rough-in day:
Bathroom Sink Plumbing Parts Checklist
- P-trap assembly, sized to match the sink's drain outlet (1.25 or 1.5 inch)
- Trap arm and wall stub-out fitting, matched to the trap diameter
- Two shutoff (angle stop) valves, one for hot and one for cold supply
- Braided supply hoses connecting the shutoff valves to the faucet
- Pop-up drain assembly or strainer, matched to the sink's drain hole
- Plumber's putty or silicone sealant for the drain flange
- PTFE (Teflon) tape for threaded connections
- Vent pipe and fittings, if not already stubbed from a wet wall
When the trap is meant to be seen
A pedestal sink or a vessel basin on an open console leaves the trap fully exposed rather than hidden in a vanity cabinet. A solid brass exposed trap kit is built for that, finished to match the faucet rather than left as bare chrome or plastic. Browse the full drains and hardware collection for matching waste and trap parts.
Most of these parts are sold separately by trap size and finish, so confirm your sink's drain hole diameter and your faucet's supply line thread (typically 3/8 inch compression in the US) before ordering, rather than assuming a universal fit.
Drain and P-Trap Sizing
This is the one section of a rough-in that actually is governed by code rather than convention, so precision matters more here than with the height figures above.
Under the International Plumbing Code, a lavatory (bathroom sink) trap must hold a water seal of not less than 2 inches and not more than 4 inches. The trap itself is commonly 1.25 inches in diameter, the minimum size IPC Table 709.1 allows for a lavatory, though 1.5 inch is also widely installed to standardize parts with a kitchen sink drain. Either is acceptable; the number that actually constrains your layout is how far that trap can sit from its vent.
| Trap Arm Diameter | Minimum Slope | Max Distance to Vent |
|---|---|---|
| 1.25 in | 1/4 in per foot | 5 ft |
| 1.5 in | 1/4 in per foot | 6 ft |
| 2 in | 1/4 in per foot | 8 ft |
These figures come from IPC/IRC Table 909.1, and I'd recommend checking the specific edition your local jurisdiction has adopted, since some municipalities amend the base code. The distance is measured from the trap weir to the vent connection, along the slope of the trap arm, and the vent connection can never sit below the trap weir regardless of the distance measured, a detail that trips up more rough-ins than the distance limit itself.
UK plumbing handles this differently. Under Approved Document H of the Building Regulations, a washbasin trap needs a minimum 32mm diameter and a 75mm water seal depth, a deeper seal than the US minimum, and there isn't a published maximum trap-to-vent distance in the same table format as the IPC. UK basin waste runs are instead sized and routed case by case against the single stack or ventilated system in use, so this is worth confirming with a Part H-qualified installer rather than applying a US distance figure directly.
Installation Overview
The sequence below assumes rough-in plumbing is already in the wall at the heights above and you're connecting the sink itself, not opening walls to relocate lines. It isn't a full step-by-step, this article covers dimensions rather than a repair walkthrough, but the order matters for anyone coordinating trades.
Installation Order
- Shut off water at both angle stops before disconnecting anything
- Set the vanity and basin, confirming the drain hole lines up with the rough-in centerline
- Install the pop-up drain assembly and P-trap, hand-tight then a quarter turn with a wrench, not over-torqued
- Connect braided supply hoses from the shutoff valves to the faucet's supply tails
- Turn water back on gradually and check every joint for drips before closing up the cabinet
- Run the tap for two minutes and recheck the P-trap connection, a slow drip often only shows under sustained flow
Confirm rough-in position against your specific faucet and sink model before final assembly. Every Insideast faucet product page includes hole spacing and connection details, and customer service can review your rough-in measurements against a specific model before you order.
How do you plumb a bathroom sink?
Rough in the hot and cold supply lines about 20 to 22 inches above the floor, spaced 4 inches to each side of the sink's centerline, then rough in the drain stub-out 18 to 20 inches above the floor on the same centerline. Once the vanity and sink are in place, connect a P-trap under the basin, vent it within the code-allowed distance for its pipe size, and tie the supply lines to the faucet with braided hoses.
What is the rough-in height for a bathroom sink?
There is no single code-mandated height. Trade convention puts the drain stub-out 18 to 20 inches above the finished floor and the supply lines 2 to 3 inches above that, around 20 to 22 inches. These figures follow standard vanity height, not a plumbing code requirement, so they can shift for a vessel sink or a custom-height vanity.
How far can a bathroom sink's P-trap be from the vent?
Under IPC/IRC Table 909.1, a 1.25 inch trap arm can run up to 5 feet to its vent, and a 1.5 inch trap arm up to 6 feet, both at a minimum slope of 1/4 inch per foot. Check the edition adopted in your local jurisdiction, since some areas amend these figures.
What size pipe is used for a bathroom sink drain?
A bathroom sink (lavatory) drain and trap is typically 1.25 inches, the minimum allowed under the IPC, though many plumbers install 1.5 inch to match kitchen sink parts and simplify future repairs. Either size is common; check what your local code and existing fixtures use before buying parts.
Do UK bathroom basins follow the same rough-in dimensions as US sinks?
Not exactly. UK plumbing doesn't use the same fixed rough-in height convention. Approved Document H instead sets a minimum 32mm waste pipe diameter and a 75mm water seal depth for basin traps. Supply and waste positions are typically set to match the specific basin and vanity rather than a standard US-style height.
Once the rough-in is confirmed, the sink and faucet are the easy part. Browse the undermount and drop-in brass sinks collection and the drains and hardware collection for matching solid brass and copper pieces, handmade in Marrakech, or add 3 or more pieces to your cart for 15% off across bath fixtures.
