Natural Gas Pipe Sizing Calculation, Step by Step

Updated: 2026-10-27

A gas project calculation makes sure every appliance gets gas at sufficient pressure without exceeding the velocity limit. You find the flow in each section, calculate its pressure drop and choose diameters until the total stays under the limit.

This guide covers 21 mbar residential installations under Turkish gas distribution company specifications and ends with a small example we solved ourselves (not one of the specifications' worked examples).

Allowed pressure drops (21 mbar)

Service box regulator, domestic regulator, meter and appliance connection losses are neglected [2], though the Kırgaz sample project gives the meter ζ = 2.0; this may vary by company [1].

1. Appliance flow rate

The net calorific value of natural gas is taken as 8250 kcal/m³ [2]. For an appliance not in the tables:

Q (m³/h) = capacity (kcal/h) / 8250 / efficiency

In İGDAŞ, efficiency is ignored below 70 kW and taken as 90% at 70 kW and above [2]. Some companies apply a flat 90% to any appliance not in the catalogue [1].

Minimum values [2]:

• Domestic cooker: 1.6 m³/h
• Combi boiler: at least 2.5 m³/h (calculated if larger)
• Instantaneous water heater: 2.2 m³/h
• Space heater: 0.7 (balanced flue) / 1.2 m³/h (open flue)

2. Diversity (simultaneity)

Because appliances do not all run at full output together, a diversity factor based on the number of units is applied to the riser and building connection. For cooker + combi, riser flows are identical in all sources [1], [3]:

Rules [3]:

• In a riser project, each unit's branch flow is at least 3.5 m³/h.
• If a flat's total exceeds 5 m³/h, the excess is added arithmetically to the flat and riser flow.
• Above 100 units, the factor for 100 units (0.397) is used.
• If commercial appliances (tea boilers, ovens, grills, etc.) are present, f = 1.

After the meter, a separate table by appliance type and count is used; for example f = 0.621 for one cooker and f = 1.0 for one combi [3]. The document gives no interpolation rule for intermediate values [3].

3. Friction loss

At 50 mbar or below [4]:

P1 − P2 = 23.2 × R × Q^1.82 / D^4.82 × L

• R = 0.6, Q in m³/h, D = pipe inner diameter (mm), L in m, P in bar absolute.
• ΔPR = (P1 − P2) × 1000 mbar.
• First section P1 = 1.021 bar (21 mbar system). Each following section's P1 is the previous P2.

Steel pipe inner diameters: DN 15 = 15.7 mm, DN 20 = 21.1 mm, DN 25 = 26.9 mm, DN 32 = 35.2 mm, DN 40 = 40.9 mm [5].

Table or formula? Kırgaz, Samgaz, Selçukgaz, Çorumgaz and similar documents require the steel flow table up to 31 m³/h; İGDAŞ, Esgaz, Palen, Torosgaz and Doğugaz use the formula at every flow [1]. The two can differ. For 3.5 m³/h in DN 25, İGDAŞ Table 9 gives 0.0234 mbar/m [5], while the formula gives about 0.0175 mbar/m.

4. Velocity check

V = 353.677 × Q / (D² × P2) (m/s) [4]

In İGDAŞ, above 50 mbar the limit is 15 m/s up to DN 65 and 25 m/s from DN 80 [6]. Kargaz and UDAŞ do not have the 25 m/s value [1].

5. Local losses (ζ)

ΔPz = 3.97 × 10⁻³ × Σζ × V² (mbar) [4]

Values from İGDAŞ Table 12; most specifications reviewed use the same set [7], [1].

6. Height effect

ΔPH = 0.049 × H (mbar) [4]

Because gas is lighter than air, pressure is gained in rising pipes: H is negative for a rise and positive for a drop [4].

7. Total, critical path and pipe size

Each section's total is ΔP = ΔPR + ΔPz + ΔPH. The sections leading to the end point with the largest loss are summed; this is the critical path [4]. If the limit is exceeded, the diameter is increased and the calculation repeated until it passes [8]. Samgaz asks for at least three decimal places [9].

Worked example (our own calculation)

One flat, 21 mbar. After the meter, a tee splits the line to a combi boiler and a cooker, giving three sections. Combi 24 kW: 24 × 860 = 20,640 kcal/h → 20,640 / 8250 = 2.50 m³/h (below 70 kW, no efficiency). Cooker 1.6 m³/h.

For section 1 we used 3.5 m³/h, the single-flat cooker + combi value. Summing by type with the post-meter table gives 0.621 × 1.6 + 1.0 × 2.5 ≈ 3.49; that summation is not written as a formula in the specification.

Section 1 (meter → tee), Q = 3.5, DN 25, L = 4 m, 1 valve + 2 elbows (Σζ = 1.3):

• 23.2 × 0.6 = 13.92; 3.5^1.82 ≈ 9.777; 26.9^4.82 ≈ 7.788 × 10⁶
• P1 − P2 = 13.92 × 9.777 / 7.788 × 10⁶ × 4 ≈ 6.99 × 10⁻⁵ bar → ΔPR ≈ 0.0699 mbar
• V = 353.677 × 3.5 / (26.9² × 1.021) ≈ 1.68 m/s
• ΔPz = 3.97 × 10⁻³ × 1.3 × 1.68² ≈ 0.0145 mbar

Section 2: Σζ = tee branch 1.3 + 2 elbows 0.8 + valve 0.5. Section 3: tee straight 0 + 2 elbows + valve. Section 2 rises 1 m to the boiler; section 3 drops 0.5 m to the cooker.

Result:

• Critical path 1 + 3 = 0.0844 + 0.1633 ≈ 0.248 mbar ≤ 0.8 mbar, passes.
• Path 1 + 2 = 0.166 mbar.
• All velocities are below 6 m/s.

If the pipe were too small: with section 1 in DN 15, ΔPR/L ≈ 0.234 mbar/m, ΔPR ≈ 0.937 mbar and V ≈ 4.92 m/s. The velocity limit holds, but this section alone exceeds 0.8 mbar, so the diameter must be increased.

How to do it with GazPlan

GazPlan's calculation engine applies these formulas to every section. Based on the selected distribution company's profile, it uses the table or formula method, the limits and the ζ values. "Auto size" first picks the smallest diameter that meets the velocity limit, then enlarges the highest-loss section on any path that exceeds its limit, one size at a time. Results go into the 12-column pipe sizing sheet and can be exported as CSV; failing sections show in red.

• Open the program
• Free pressure drop and flow calculators
• Calculation differences by distribution company

For the drawing side, see how to draw a natural gas installation project.

This guide is for information only; projects are prepared by a certified engineer and the distribution company's current specification prevails.

Sources

1. GazPlan specification comparison (13 Turkish gas distribution company specifications), 2026, sections 2–4.
2. İGDAŞ Doğal Gaz İç Tesisat Teknik Esasları (Technical Principles) 2017, pp. 17–18.
3. İGDAŞ 2017, pp. 18, 20–21 (Tables 7 and 8).
4. İGDAŞ 2017, p. 19 (clause 4.4.1).
5. İGDAŞ 2017, p. 15 (Table 1), p. 22 (Table 9).
6. İGDAŞ 2017, pp. 18–19.
7. İGDAŞ 2017, pp. 23–24 (Table 12, Form 1).
8. İGDAŞ 2017, p. 20.
9. SAMGAZ İç Tesisat Şartnamesi (Internal Installation Specification), Rev. 2019.03, p. 224.