What Is Undermount Cutout? Definition & Guide
Quick Definition
An undermount cutout is an opening fabricated in a stone countertop to accommodate a sink that mounts from below the stone surface. Unlike drop-in sinks that sit on top of the counter, undermount sinks are bonded to the underside of the stone, leaving the polished stone edge visible around the entire opening. The cutout must be precisely sized, positioned, and edge-finished for a proper undermount installation.
TL;DR
- Undermount cutout is the hole cut in a countertop for a sink that mounts from below
- The stone edge around the cutout is exposed and must be polished
- Cutout dimensions are typically 1/4"-3/8" larger than the sink bowl per manufacturer specs
- Requires precise template measurements - errors here cause costly remakes
- Inner edge is eased, bullnosed, or flat-polished depending on design preference
- CNC cutting produces the most accurate and consistent cutouts
- SlabWise's template verification catches dimensional errors before fabrication
How Undermount Cutouts Differ from Drop-In Cutouts
The distinction matters for fabrication because the finishing requirements are completely different:
| Feature | Undermount Cutout | Drop-In Cutout |
|---|---|---|
| Sink position | Below the stone | Sits on top, rim covers the edge |
| Edge visibility | Fully visible, must be polished | Hidden under the sink rim |
| Edge finishing | Eased, polished, or profiled | Rough cut acceptable |
| Dimensional tolerance | Tight (±1/16") | Moderate (±1/8") |
| Template accuracy needed | High | Moderate |
| Fabrication time | More (edge finishing) | Less |
| Typical cost per cutout | $150-$350 | $50-$100 |
The key takeaway: undermount cutouts cost more to fabricate because the stone edge is the finished product. Every imperfection is visible.
Cutout Sizing and Dimensions
Getting the cutout size right depends on the sink manufacturer's specifications and the stone thickness.
Standard Sizing Rules
- Cutout width = sink bowl width + reveal (typically 1/4" to 3/8" on each side)
- Cutout length = sink bowl length + reveal
- Corner radius = must match the sink bowl corner radius
- Reveal = the amount of polished stone edge visible between the cutout and the sink rim
Common Sink Types and Cutout Considerations
| Sink Type | Typical Cutout Size | Special Considerations |
|---|---|---|
| Single bowl | 28"-33" x 16"-20" | Center on base cabinet |
| Double bowl | 30"-36" x 16"-20" | Divider bar strength matters |
| Farmhouse/apron | 30"-36" x 18"-22" | Front of countertop removed for apron |
| Bar sink | 12"-18" x 12"-16" | Placement on narrow sections |
| Prep sink | 14"-20" x 14"-18" | Often in islands, tight nesting |
The Reveal Question
The "reveal" is how much polished stone edge is visible between the cutout opening and where the sink bowl begins below. There are three common approaches:
- Positive reveal (most common) - the cutout is slightly smaller than the sink, so the stone overhangs the bowl by 1/4"-3/8". You see a small stone ledge all around.
- Zero reveal - the cutout matches the sink bowl exactly. The stone edge and sink edge are flush.
- Negative reveal - the cutout is larger than the sink, so the sink bowl is visible beyond the stone edge. Less common and requires careful sizing.
Most undermount installations use a positive reveal. It's the most forgiving in terms of alignment tolerance and creates a clean finished look.
Fabrication Process
Step 1: Template Measurement
The template must capture:
- Exact sink position relative to cabinets and walls
- Sink model and manufacturer (for cutout dimensions)
- Desired reveal amount
- Faucet hole positions
- Any accessories (soap dispenser, air gap, etc.)
Step 2: CNC Programming
The cutout path is programmed with:
- Precise dimensions per the sink manufacturer's template
- Corner radii matching the sink bowl
- Any offset for the selected reveal type
- Faucet and accessory hole locations
Step 3: Cutting
CNC routers or waterjets cut the opening:
- CNC router - cuts the shape with a diamond bit, can leave a relatively smooth edge
- Waterjet - cuts with high-pressure water and abrasive, minimal kerf, very precise
- Bridge saw - can cut rectangular openings but struggles with radiused corners
Step 4: Edge Finishing
The inner edge of the cutout is finished to the specified profile:
- Eased - lightly softened corners, most common
- Flat polished - smooth, straight edge with polished faces
- Half bullnose - rounded inner edge, softer look
- Polished through progressive grits (50 through 3000+) to match the countertop surface
Step 5: Quality Check
Before the countertop leaves the shop:
- Dimensions are verified against the sink manufacturer's template
- Corner radii are checked
- Edge polish is inspected for consistency
- Faucet holes are verified for position and diameter
Common Undermount Cutout Errors
These are the mistakes that lead to remakes:
1. Wrong Cutout Size
Using outdated or incorrect sink manufacturer templates. Sink models change, and a cutout sized for last year's model may not fit this year's.
Prevention: Always confirm the exact sink model with the customer and download the current manufacturer template.
2. Misplaced Cutout
The sink is off-center on the cabinet, or too close to the back wall, or misaligned with the base cabinet opening.
Prevention: Template measurement must reference the actual cabinet position, not assumed dimensions from a floor plan.
3. Incorrect Corner Radii
The cutout has sharp corners when the sink has rounded corners (or vice versa). This creates visible gaps at the corners that are difficult to hide.
Prevention: Match corner radii to the sink manufacturer's spec. CNC machines handle this automatically when properly programmed.
4. Rough Edge Finish
The polished inner edge has visible grinding marks, uneven polish, or inconsistent easing.
Prevention: Follow the full polishing sequence. Don't skip grit stages. Inspect under strong lighting.
5. Cracked Countertop at Cutout
The stone cracks during cutting because the cutout weakens a narrow section of the slab (common when the cutout is close to a seam or the slab edge).
Prevention: Maintain minimum material widths between the cutout and any edge. Most fabricators require 3"+ of stone between the cutout and any slab edge.
Template Verification for Undermount Cutouts
Undermount cutouts are one of the highest-risk areas in countertop fabrication because errors are immediately visible and almost never repairable. A cutout that's 1/2" off in any direction means the sink doesn't fit properly, the reveal is uneven, or the cutout is visible beyond the sink edge.
SlabWise's Template Verification system runs a 3-layer check on every template:
- Dimensional verification - cutout size matches the specified sink model
- Position verification - cutout location matches cabinet layout
- Clearance verification - adequate stone remains between the cutout and all edges/seams
This catches errors before any stone is cut, preventing remakes that typically cost $1,500-$4,000 per occurrence.
Pricing Undermount Cutouts
| Component | Typical Cost Range |
|---|---|
| CNC cutout (straight + radiused) | $75-$150 |
| Edge polishing (per cutout) | $50-$125 |
| Faucet hole(s) | $25-$50 each |
| Accessory holes (soap, air gap) | $15-$30 each |
| Total per single-bowl undermount | $150-$350 |
Double-bowl sinks are typically priced as two separate cutouts since each bowl requires its own cut and edge finishing.
FAQ
What is an undermount cutout?
An undermount cutout is the opening fabricated in a stone countertop for a sink that mounts from below the stone. The stone edge around the opening is polished and visible in the finished installation.
How is an undermount cutout sized?
The cutout is sized based on the sink manufacturer's template, typically 1/4" to 3/8" larger than the sink bowl on each side to create a positive reveal of polished stone.
What does the edge of an undermount cutout look like?
The inner edge is polished to match the countertop surface. Most undermount cutouts have an eased (slightly softened) or flat-polished edge. Some customers request a half-bullnose inner edge for a softer look.
How much does an undermount cutout cost?
A standard single-bowl undermount cutout typically costs $150-$350 including the CNC cut and edge polishing. Faucet and accessory holes are additional.
Can any sink be undermounted?
Most stainless steel, composite, and cast iron sinks can be undermounted in stone countertops. However, the sink must be specifically designed for undermount installation - not all sinks support this mounting method.
How is an undermount sink attached to the stone?
Undermount sinks are bonded to the underside of the stone with silicone adhesive and secured with mechanical clips or brackets that hook over the sink rim and fasten to the countertop's underside.
What happens if the cutout is the wrong size?
If the cutout is too large, the sink won't cover it and the opening is visible. If too small, the sink won't fit. Either case typically requires remaking the countertop section - which is why template accuracy matters so much.
Can undermount cutouts be done on 2cm stone?
Yes, but 2cm stone is thinner and has less material around the cutout for structural support. Most fabricators recommend 3cm stone for undermount sinks, or reinforcing 2cm stone with a support frame.
How close can an undermount cutout be to the countertop edge?
Most fabricators require a minimum of 3 inches of stone between the cutout and any countertop edge. Less than that increases the risk of cracking during fabrication or installation.
Do faucet holes get drilled at the same time as the cutout?
Yes. Faucet holes are typically drilled during the same CNC operation as the sink cutout, using the positions specified in the template. This ensures precise alignment relative to the sink.
Verify Every Cutout Before You Cut
Undermount cutout errors are among the most expensive mistakes in countertop fabrication. SlabWise's template verification catches size, position, and clearance errors before stone is touched.
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Sources
- Natural Stone Institute - Sink Cutout Fabrication Standards
- Marble Institute of America - Undermount Sink Installation Guidelines 3.?"National Kitchen and Bath Association - Sink Placement Specifications
- Blanco - Undermount Sink Template and Installation Guide
- Kohler - Undermount Sink Dimensional Specifications
- Stone World Magazine - "Getting Sink Cutouts Right Every Time"