Model Rocket Body Tube Metric Equivalent Chart

Understanding Model Rocket Body Tube Sizing

Model rocket body tubes use a historical designation system (BT-5, BT-20, BT-50, etc.) that can be confusing for builders working in metric units. This comprehensive guide provides exact metric equivalents for all standard body tube sizes, helping international rocketeers and 3D printing enthusiasts work with precise measurements.

Model rocket body tube metric conversion chart with BT sizes and millimeter equivalents

Reference table showing Estes BT-5 through BT-101 sizes converted to millimeters with wall thickness

The BT Numbering System

The BT (Body Tube) numbering system originated with Estes Industries in the 1960s. The numbers don’t directly correspond to dimensions—they’re simply sequential designations. This creates confusion when trying to determine actual sizes or convert to metric measurements.

Key facts:

BT numbers are arbitrary designations, not measurements
Most tubes are based on imperial (inch) dimensions
Metric equivalents are calculated conversions, not original specifications
Wall thickness varies between tube sizes

Complete BT to Metric Conversion Table

Small Diameter Tubes (Under 30mm)

BT Size	Inner Diameter (inches)	Inner Diameter (mm)	Outer Diameter (inches)	Outer Diameter (mm)	Wall Thickness (inches)	Wall Thickness (mm)	Common Use
BT-3	0.349″	8.86 mm	0.375″	9.53 mm	0.013″	0.33 mm	Micro rockets
BT-5	0.515″	13.08 mm	0.541″	13.74 mm	0.013″	0.33 mm	Mini engines
BT-20	0.710″	18.03 mm	0.736″	18.69 mm	0.013″	0.33 mm	18mm engines
BT-30	0.725″	18.42 mm	0.765″	19.43 mm	0.021″	0.53 mm	Specialty
BT-40	0.765″	19.43 mm	0.821″	20.85 mm	0.023″	0.58 mm	Specialty
BT-48	0.902″	22.91 mm	0.928″	23.57 mm	0.013″	0.33 mm	Specialty

Medium Diameter Tubes (30-60mm)

BT Size	Inner Diameter (inches)	Inner Diameter (mm)	Outer Diameter (inches)	Outer Diameter (mm)	Wall Thickness (inches)	Wall Thickness (mm)	Common Use
BT-50	0.950″	24.13 mm	0.976″	24.79 mm	0.013″	0.33 mm	24mm engines
BT-51	0.984″	24.99 mm	1.011″	25.68 mm	0.013″	0.33 mm	Specialty
BT-52	0.988″	25.10 mm	1.114″	28.30 mm	0.063″	1.60 mm	Heavy wall
BT-55	1.283″	32.59 mm	1.325″	33.66 mm	0.021″	0.53 mm	29mm engines
BT-56	1.304″	33.12 mm	1.346″	34.19 mm	0.021″	0.53 mm	Specialty
BT-58	1.498″	38.05 mm	1.540″	39.12 mm	0.021″	0.53 mm	38mm engines
BT-60	1.595″	40.51 mm	1.637″	41.58 mm	0.021″	0.53 mm	Standard
BT-63	1.639″	41.63 mm	1.681″	42.70 mm	0.021″	0.53 mm	Specialty

Large Diameter Tubes (60-100mm)

BT Size	Inner Diameter (inches)	Inner Diameter (mm)	Outer Diameter (inches)	Outer Diameter (mm)	Wall Thickness (inches)	Wall Thickness (mm)	Common Use
BT-67	2.000″	50.80 mm	2.040″	51.82 mm	0.020″	0.51 mm	2-inch tubes
BT-70	2.175″	55.25 mm	2.217″	56.31 mm	0.021″	0.53 mm	Standard large
BT-80	2.560″	65.02 mm	2.600″	66.04 mm	0.021″	0.53 mm	Standard large
BT-99	3.668″	93.17 mm	3.790″	96.27 mm	0.021″	0.53 mm	4-inch tubes
BT-100	3.702″	94.31 mm	3.744″	95.10 mm	0.021″	0.53 mm	Specialty
BT-101	3.896″	98.96 mm	3.938″	100.03 mm	0.021″	0.53 mm	4-inch tubes

Most Common BT Sizes and Metric Equivalents

For practical rocketry, these are the most frequently used sizes:

BT-5 (13mm)

Metric equivalent: 13mm inner, 13.7mm outer
Use: Mini engines (13mm motors)
Common rockets: Micro Maxx, Mini Maxx

BT-20 (18mm)

Metric equivalent: 18mm inner, 18.7mm outer
Use: Standard 18mm engines (A-C motors)
Common rockets: Alpha, Wizard, Big Bertha

BT-50 (24mm)

Metric equivalent: 24mm inner, 24.8mm outer
Use: Standard 24mm engines (D-G motors)
Common rockets: Mean Machine, Nike Smoke

BT-55 (33mm)

Metric equivalent: 33mm inner, 33.7mm outer
Use: 29mm engines, payload sections
Common rockets: Mercury Redstone, Saturn V

BT-60 (41mm)

Metric equivalent: 41mm inner, 41.6mm outer
Use: Large model rockets, clusters
Common rockets: Big Bertha (large), Patriot

BT-70 (56mm)

Metric equivalent: 56mm inner, 56.3mm outer
Use: Mid-power rockets, 38mm engines
Common rockets: LOC IV, various mid-power

BT-80 (66mm)

Metric equivalent: 66mm inner, 66mm outer
Use: High-power rockets, 54mm engines
Common rockets: LOC V, various HPR

Metric Tube Standards

Some manufacturers produce tubes in true metric sizes, particularly for international markets and 3D printing applications.

Common Metric Sizes

Metric Size	Inner Diameter	Outer Diameter	BT Equivalent	Use
13mm	13.0mm	14.0mm	BT-5	Mini rockets
18mm	18.0mm	19.0mm	BT-20	Standard small
24mm	24.0mm	25.0mm	BT-50	Standard medium
29mm	29.0mm	30.0mm	Near BT-55	Standard mid
38mm	38.0mm	39.0mm	BT-58	Mid-power
54mm	54.0mm	55.0mm	Near BT-70	High-power
75mm	75.0mm	76.0mm	Near BT-80	Large HPR
98mm	98.0mm	99.0mm	BT-101	Very large HPR

3D Printing Considerations

When designing 3D printed rocket components, metric dimensions are often easier to work with than imperial BT sizes.

Design Recommendations

For body tubes:

Use true metric sizes (18mm, 24mm, 29mm) for easier design
Account for printer tolerance (±0.2mm typical)
Design couplers with 0.2-0.3mm clearance

For motor mounts:

Use exact motor dimensions (18mm, 24mm, 29mm, 38mm, 54mm)
Add 0.3-0.5mm clearance for motor insertion
Consider heat resistance near motor

For nose cones:

Match outer diameter to body tube outer diameter
Add 0.2-0.3mm clearance for shoulder fit
Consider wall thickness (1.5-2.0mm typical)

Tolerance Stack-Up

When mixing BT-sized components with metric 3D printed parts:

Component	Tolerance	Fit Type
BT-50 tube	±0.1mm	Standard
3D printed coupler	±0.2mm	Snug fit
3D printed nose cone	±0.2mm	Slip fit
3D printed fin can	±0.3mm	Press fit

Recommendation: Design for slip fits (0.2-0.3mm clearance) rather than press fits to accommodate tolerance variations.

International Sizing

European Standards

European rocketry often uses true metric sizing:

18mm: Equivalent to BT-20
24mm: Equivalent to BT-50
29mm: Between BT-55 and BT-58
38mm: Equivalent to BT-58
54mm: Between BT-70 and BT-80

Asian Standards

Asian manufacturers typically follow metric standards:

13mm: Mini rockets
18mm: Standard small
24mm: Standard medium
30mm: Mid-power
40mm: Large models

Conversion Formulas

Inches to Millimeters

Formula: mm = inches × 25.4

Examples:

0.976″ × 25.4 = 24.79mm (BT-50 outer)
1.637″ × 25.4 = 41.58mm (BT-60 outer)
2.217″ × 25.4 = 56.31mm (BT-70 outer)

Millimeters to Inches

Formula: inches = mm ÷ 25.4

Examples:

24mm ÷ 25.4 = 0.945″ (near BT-50)
29mm ÷ 25.4 = 1.142″ (between BT-55 and BT-58)
54mm ÷ 25.4 = 2.126″ (near BT-70)

Wall Thickness Standards

Standard Wall Thicknesses

Thickness (inches)	Thickness (mm)	Application
0.013″	0.33mm	Light duty, small rockets
0.020″	0.51mm	Standard duty
0.021″	0.53mm	Standard duty (most common)
0.023″	0.58mm	Heavy duty
0.030″	0.76mm	Heavy duty, specialty
0.063″	1.60mm	Extra heavy, reinforced

Wall Thickness Selection Guide

For low-power rockets (A-D motors):

Use 0.013″ (0.33mm) for weight savings
Adequate for typical flight loads

For mid-power rockets (E-G motors):

Use 0.021″ (0.53mm) standard wall
Good balance of strength and weight

For high-power rockets (H+ motors):

Use 0.030″ (0.76mm) or thicker
Consider reinforced or composite materials

Practical Applications

Substituting Metric Tubes for BT Sizes

When exact BT sizes aren’t available, metric tubes can substitute:

BT Size	Metric Substitute	Fit Quality	Notes
BT-20	18mm metric	Excellent	Nearly identical
BT-50	24mm metric	Good	Slightly smaller
BT-55	29mm metric	Fair	Smaller, may need adapter
BT-60	40mm metric	Good	Slightly smaller
BT-70	54mm metric	Fair	Smaller, different motor mount

Motor Mount Compatibility

Motor Size	BT Size	Metric Equivalent	Notes
13mm	BT-5	13mm	Direct fit
18mm	BT-20	18mm	Direct fit
24mm	BT-50	24mm	Direct fit
29mm	BT-55/58	29mm	Use 29mm metric tube
38mm	BT-58	38mm	Direct fit
54mm	BT-70/80	54mm	Use 54mm metric tube

Measurement Tools

Calipers

Digital calipers provide the most accurate measurements:

Resolution: 0.01mm or 0.001″
Accuracy: ±0.02mm typical
Measure both inner and outer diameters

Tube Gauges

Specialized tube gauges quickly identify BT sizes:

Insert tube into gauge
Read BT size directly
Less accurate than calipers but faster

3D Printed Test Fits

Print test rings to verify fit:

Design rings at various diameters
Test fit with actual components
Adjust CAD models based on results

Common Conversions for Quick Reference

Small Tubes

BT-5 = 13mm
BT-20 = 18mm
BT-50 = 24mm

Medium Tubes

BT-55 = 33mm
BT-58 = 38mm
BT-60 = 41mm

Large Tubes

BT-70 = 56mm
BT-80 = 66mm
BT-101 = 98mm

Conclusion

Understanding metric equivalents for BT-sized tubes is essential for international rocketeers, 3D printing enthusiasts, and anyone working with metric measurement systems. While BT sizes are historical designations, the metric conversions are straightforward and allow for precise component design and sourcing.

Key points:

BT numbers are arbitrary, not measurements
Most common sizes: BT-20 (18mm), BT-50 (24mm), BT-60 (41mm)
Metric tubes are often easier for 3D printing
Account for tolerances when mixing systems
Use calipers for accurate measurements

For 3D printing applications, true metric sizes (18mm, 24mm, 29mm, 38mm, 54mm) are often more practical than exact BT equivalents.

Quick Reference Card:

Related Guides

Resources

Estes Industries

BT-5: 13mm
BT-20: 18mm
BT-50: 24mm
BT-55: 33mm
BT-60: 41mm
BT-70: 56mm
BT-80: 66mm

Pillar: This article is part of the Body Tubes topic hub. See all articles in this section for related comparisons and guides.