MM To Inches The Ultimate Guide
MM to Inches Conversion for Wood Stove Flue Sizes
Convert common wood-burning stove flue sizes from millimetres to inches, compare nominal metric and imperial sizes and understand what to check before ordering a flexible liner, stove pipe, adaptor or chimney cowl.
How do you convert millimetres to inches?
Divide the measurement in millimetres by 25.4. For example, 150 ÷ 25.4 = 5.91 inches. In the stove and chimney industry, 150 mm is commonly described as the nominal metric equivalent of a 6-inch flue, but nominal metric and imperial components should not be assumed to be mechanically interchangeable without checking the product specification.
Interactive Conversion Tool
Convert stove flue sizes between mm and inches
Enter a measurement, select the unit and the calculator will provide the exact converted value together with the nearest commonly used nominal stove-flue size.
The result is a mathematical conversion only. Always order flue components by their stated system diameter and manufacturer’s compatibility information.
150 mm ÷ 25.4 = 5.91 inches
Guide Navigation
Wood stove flue conversion guide
Quick Reference
MM to inches conversion table for stove flues
These are exact mathematical conversions followed by the closest commonly referenced nominal imperial flue size.
| Metric size | Exact conversion | Common nominal description | Typical context |
|---|---|---|---|
| 100 mm | 3.94 inches | Approximately 4 inches | Small or specialist appliances and selected flue systems |
| 125 mm | 4.92 inches | Approximately 5 inches | Many compact stoves where specifically permitted |
| 150 mm | 5.91 inches | Approximately 6 inches | Widely used wood-burning stove and liner size |
| 180 mm | 7.09 inches | Approximately 7 inches | Larger appliances and selected fireplace arrangements |
| 200 mm | 7.87 inches | Approximately 8 inches | Large appliances, inglenooks and some open fires |
| 220 mm | 8.66 inches | Approximately 8¾ inches, often grouped near 9 inches | Large or specialist flue and fireplace applications |
Do not simply round a component size up or down
“150 mm” and “6 inch” are commonly treated as equivalent nominal sizes, but a precisely manufactured metric part may not fit an imperial component from another system. Use approved adaptors and confirm the declared internal or external diameter before ordering.
Reverse Conversion
Inches to millimetres for stove flues
Multiply inches by 25.4 to calculate the exact metric equivalent. Nominal flue sizes are shown separately.
| Imperial size | Exact metric conversion | Common nominal metric size |
|---|---|---|
| 4 inches | 101.6 mm | Often associated with 100 mm systems |
| 5 inches | 127 mm | Often associated with 125 mm systems |
| 6 inches | 152.4 mm | Often associated with 150 mm systems |
| 7 inches | 177.8 mm | Often associated with 180 mm systems |
| 8 inches | 203.2 mm | Often associated with 200 mm systems |
| 9 inches | 228.6 mm | Not the exact equivalent of 220 mm |
Flue Performance
Why accurate wood stove flue sizing matters
The diameter influences flue-gas velocity, temperature, resistance and draught. It is part of a complete system that also includes the appliance, chimney height, route, insulation and terminal.
Supports stable chimney draught
A correctly designed flue helps combustion products rise through the chimney. A flue that is unsuitable for the appliance can contribute to difficult lighting, unstable combustion or smoke spillage.
Maintains flue-gas temperature
Excessive cooling can encourage condensation, soot and tar deposits. Diameter, insulation and chimney route all affect the rate at which the gases cool.
Preserves appliance compatibility
The stove manufacturer specifies a flue outlet and installation requirements. Changing diameter without an approved design can undermine the tested appliance arrangement.
Allows components to connect correctly
Stove pipe, adaptors, flexible liners, twin-wall flues, pot hangers and chimney cowls must form a compatible system with properly fitted joints.
Reduces avoidable deposit formation
Incorrect diameter can contribute to slow or cooled flue gases. Deposits are also affected by fuel moisture, firing method, appliance operation and chimney insulation.
Supports regulatory compliance
UK installations should follow the appliance instructions, applicable standards and the requirements of Approved Document J or the relevant national guidance.
Diameter Comparison
Compare common stove flue diameters
The visual below is illustrative rather than a full-scale measuring template. Measure the actual product or system before ordering.
Size-by-Size Guide
Detailed breakdown of common wood stove flue sizes
The examples below are general. The correct diameter is determined by the appliance instructions and the design of the complete chimney system, not simply by room size or heat output.
100 mm Exact conversion: 3.94 inches
A 100 mm flue is a compact diameter found on selected small, specialist or non-standard appliances. It should not be assumed suitable for a domestic wood-burning stove unless the appliance and installation requirements specifically permit it.
- May be used on selected compact or specialist appliances.
- Component availability can be more limited.
- Small diameter does not automatically guarantee strong draw.
- Do not reduce a larger stove outlet to 100 mm without approval.
125 mm Exact conversion: 4.92 inches
A 125 mm or nominal 5-inch flue is used by many compact modern appliances where the manufacturer’s instructions and applicable requirements permit the size.
- Common on selected compact and approved stove models.
- Frequently paired with 125 mm flexible liner.
- The stove specification must expressly permit the diameter.
- Requires a compatible 125 mm terminal or adaptor.
150 mm Exact conversion: 5.91 inches
A 150 mm or nominal 6-inch flue is one of the most widely used sizes for domestic wood-burning and multi-fuel stove installations in the UK.
- Widely available in stove pipe and flexible liner systems.
- Common on medium domestic wood-burning stoves.
- Broad choice of liner cowls and chimney terminals.
- Exact compatibility must still be checked by system.
180 mm Exact conversion: 7.09 inches
A 180 mm flue may be specified for larger appliances, selected boiler stoves, fireplaces or systems with greater flue-gas volume. It should be treated as a distinct metric size rather than assumed identical to every 7-inch component.
- Used on selected larger-output appliances.
- Requires correctly sized pipe, liner and terminal components.
- Chimney height and insulation remain important.
- Oversizing can allow gases to cool excessively.
200 mm Exact conversion: 7.87 inches
A 200 mm or nominal 8-inch flue is associated with selected large appliances, fireplaces, inglenooks and open-fire applications. The larger diameter must form part of a properly designed chimney.
- Provides greater cross-sectional outlet area.
- May suit large appliances or fireplace openings.
- Requires an appropriately sized chimney cowl.
- Large diameter alone does not guarantee adequate draw.
220 mm Exact conversion: 8.66 inches
A 220 mm flue is a large metric size for specialist domestic, fireplace or custom chimney arrangements. It is not the exact equivalent of a 9-inch flue, which measures 228.6 mm.
- Used in selected large or specialist installations.
- Component choice may be less standardised.
- Requires careful matching of adaptors and terminals.
- Professional system design is advisable.
Selection Guide
How to choose the correct stove flue size
The appliance manufacturer’s instructions are the starting point. Do not select a liner solely because it is the closest rounded conversion in inches.
Check the appliance data plate and manual
Identify the stated flue outlet diameter, permitted chimney arrangement and any conditions governing the use of a smaller or larger liner.
- Locate the declared flue outlet size.
- Check whether the measurement is internal or external.
- Confirm the appliance fuel and operating category.
- Retain the instructions with the installation records.
Identify the complete flue system
Establish whether the installation uses vitreous-enamel stove pipe, flexible liner, rigid liner, twin-wall insulated flue or a combination of approved components.
- Do not mix incompatible proprietary systems.
- Check male and female joint orientation.
- Use the correct connecting adaptor.
- Maintain access for sweeping and inspection.
Assess chimney height and route
Draught is influenced by vertical height, bends, offsets, exposure, terminal position, insulation and the temperature of the flue gases.
- Minimise unnecessary changes of direction.
- Keep external flue sections properly insulated.
- Consider nearby roofs, trees and structures.
- Diagnose existing smoke problems before altering the cowl.
Confirm regulatory and installer requirements
A competent installer should confirm the flue size, system components, termination and commissioning requirements for the specific property.
- Follow the stove manufacturer’s instructions.
- Check applicable UK building guidance.
- Use declared and compatible flue components.
- Commission and test the completed installation.
Component Compatibility
Matching pipe, liner, adaptors and chimney terminals
A converted measurement does not confirm that two components will connect. Check nominal diameter, actual dimensions and the declared system connection.
| Component | What to measure | Common sizing error | Best practice |
|---|---|---|---|
| Stove outlet | Declared appliance outlet diameter | Measuring only the visible collar externally | Use the appliance manual and approved connector |
| Vitreous-enamel stove pipe | Nominal system diameter and joint type | Mixing components from different systems | Use matching pipe and fittings from a compatible range |
| Flexible flue liner | Nominal internal diameter and product specification | Assuming 150 mm and 6 inches are physically identical | Order compatible liner, adaptor and terminal components |
| Twin-wall flue | Internal bore and external casing diameter | Ordering by the larger external diameter | Use the stated system bore and proprietary adaptors |
| Pot hanger | Liner size, pot dimensions and support arrangement | Choosing only by chimney-pot diameter | Match both the liner and chimney termination |
| Chimney cowl | Liner connection or chimney-pot fitting range | Confusing pot-mounted and liner-mounted cowls | Select by fuel, flue size and fixing method |
Terminal Selection
Choosing a chimney cowl for each flue size
A chimney cowl should be selected by fuel, appliance, flue or liner diameter, fixing type and the problem being addressed. Diameter alone is not enough.
UK Installation Guidance
Stove flue sizes and UK Building Regulations
Flue sizing cannot safely be reduced to a generic “small stove equals 5 inches” rule. The appliance instructions and complete installation design take priority.
Follow the appliance instructions
The stove manufacturer identifies the intended outlet and may state conditions for the chimney diameter, minimum height, fuel and connecting components.
Check current national guidance
In England, Approved Document J provides guidance for combustion appliances and fuel-storage systems. Scotland, Wales and Northern Ireland have their own applicable building guidance.
Do not reduce the flue casually
Reducing the diameter can increase resistance and may conflict with the appliance specification. Any transition should use an approved adaptor and form part of a compliant system design.
Test the completed installation
The completed stove and chimney should be inspected, tested and commissioned. Smoke draw, ventilation, termination and carbon monoxide alarm provision should all be addressed.
Nominal conversion is not regulatory approval
Knowing that 125 mm is approximately 5 inches or that 150 mm is approximately 6 inches does not confirm that the size is permitted for a particular stove. Verify the appliance instructions and obtain competent installation advice.
Avoid Ordering Errors
Four checks before buying flue components
A few minutes spent confirming the system can prevent incompatible liners, adaptors, pipes and chimney cowls being ordered.
Identify the stated size
Check whether the product is sold as a metric or imperial system and whether the stated measurement refers to the internal or external diameter.
Convert accurately
Divide millimetres by 25.4 or multiply inches by 25.4. Keep the exact value separate from the nominal trade description.
Check the connection
Confirm whether the component is push-fit, screw-fit, male, female, pot-mounted or liner-mounted.
Verify the complete system
Ensure the stove, pipe, adaptor, liner, support and cowl are compatible and suitable for the intended fuel.
Frequently Asked Questions
MM to inches and stove flue size FAQs
What is the formula for converting mm to inches?
Divide the measurement in millimetres by 25.4. For example, 125 ÷ 25.4 = 4.92 inches.
Is 100 mm the same as 4 inches?
Not exactly. Four inches equals 101.6 mm. A 100 mm flue is nevertheless often described as the nominal metric equivalent of a 4-inch system.
Is 125 mm the same as 5 inches?
Not exactly. Five inches equals 127 mm, while 125 mm equals 4.92 inches. The two are commonly associated as nominal trade sizes, but component compatibility should be checked.
Is 150 mm the same as 6 inches?
Six inches equals 152.4 mm, while 150 mm equals 5.91 inches. A 150 mm flue is commonly described as a nominal 6-inch flue, but exact component dimensions can differ.
What is 180 mm in inches?
A 180 mm flue is 7.09 inches. It is commonly grouped with nominal 7-inch flue systems, although 7 inches is exactly 177.8 mm.
What is 200 mm in inches?
A 200 mm flue is 7.87 inches. It is commonly described as the nominal metric equivalent of an 8-inch flue.
What is 220 mm in inches?
A 220 mm flue is 8.66 inches. It is not the exact equivalent of 9 inches, which is 228.6 mm.
Can I connect 6-inch stove pipe to a 150 mm liner?
Only where the components and adaptor are declared compatible. Although the sizes are commonly treated as nominal equivalents, exact dimensions, joint types and manufacturing tolerances can vary.
Can I reduce a 150 mm stove outlet to 125 mm?
Do not reduce the outlet unless the stove manufacturer’s instructions and applicable installation requirements expressly permit the arrangement. An unauthorised reduction may restrict the flue and affect appliance operation.
Does a larger flue improve chimney draught?
Not necessarily. An excessively large flue can slow and cool the gases, while an undersized flue can create excessive resistance. Diameter must be considered with chimney height, insulation, route, appliance output and flue-gas temperature.
Is a 125 mm flue suitable for every small wood stove?
No. The correct size is specified by the appliance manufacturer and applicable installation guidance. Some compact appliances use 125 mm flues, while others require 150 mm or another diameter.
What chimney cowl fits a 150 mm flue liner?
Use a chimney liner cowl declared for a 150 mm liner and suitable for the appliance and fuel. Confirm whether the liner requires a push-fit, screw-fit or pot-hanging terminal.
Should I measure the inside or outside of a flue pipe?
Use the manufacturer’s stated nominal system diameter. Physical measurement alone can be misleading because some products are described by their internal bore while connecting ends and outer casings have different dimensions.
What is the most common wood stove flue size in the UK?
A 150 mm or nominal 6-inch flue is widely used for domestic wood-burning stoves. Many selected modern appliances also use 125 mm systems where specifically permitted.
Where can I buy flexible stove flue liners?
Browse flexible flue liners at Hot Box Stoves and check the appliance, liner diameter and complete installation requirements before ordering.
Measure Once, Order Correctly
Match your stove, flue liner and chimney cowl
Use the conversion tool to understand the metric and imperial measurements, then confirm the stated appliance outlet, liner system, connection type and chimney-terminal requirements before purchasing.
About this flue-size guide
This page provides general metric and imperial conversion information for common wood stove, chimney liner and flue sizes.
Mathematical conversions do not confirm that components are physically interchangeable or suitable for a particular appliance. Follow the appliance manufacturer’s instructions and use compatible, declared flue-system components.
Stove and chimney installations should be designed, fitted and commissioned by an appropriately competent person.
Last reviewed: July 2026.