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90° vs 45° Ducting Bends: The Hidden Airflow Drop Explained

A 90° bend in cooker hood ducting can reduce airflow by up to 50% — far more than most people realise when planning an installation. Replacing a sharp 90° turn with two shallower 45° bends cuts turbulence significantly, keeps extraction performance closer to the hood’s rated output, and reduces operating noise. Understanding this single detail can make the difference between a hood that performs as specified and one that struggles from day one.

50% Max airflow loss from a single 90° bend in short duct runs

2× Pressure drop of a 90° bend vs two 45° bends

1.5m Equivalent straight duct added by each 90° elbow (150mm diameter)

3m Recommended maximum total duct length for most domestic hoods

Why Ducting Bends Matter More Than You Think

When a cooker hood is tested for its rated extraction performance — quoted in cubic metres per hour (m³/h) — that figure is measured under laboratory conditions with minimal or no ducting resistance. The moment you run real ductwork through a kitchen wall or ceiling, that number starts to fall. Duct length, diameter, surface texture, and the number of bends all add what engineers call resistanceResistance (or pressure drop) is the friction and turbulence that opposing forces place on airflow, measured in Pascals (Pa). More resistance means the motor works harder for less airflow output. to the system.

Of all these factors, bends are the most commonly underestimated. A straight metre of smooth rigid ducting adds relatively modest resistance. A single elbow at 90° can add the equivalent of 1–1.5 metres of additional straight pipe — and in a short duct run, that proportion is enormous.

For installers and homeowners alike, the practical consequence is straightforward: a hood rated at 600 m³/h might deliver only 350–400 m³/h through a poorly routed duct run with two or three sharp bends. The motor runs louder trying to compensate, grease filters clog faster, and cooking smells linger longer than they should.

The Physics: What Happens at a Bend

Air flowing through a duct behaves predictably when the path is straight: it moves in parallel streams with minimal internal friction. A bend disrupts this. As airflow hits a change in direction, it separates from the outer wall of the elbow, creating turbulence and recirculation zonesAreas where airflow doubles back on itself rather than moving forward, dramatically increasing energy loss at that point in the duct. that rob the system of energy.

The sharper the bend, the more violent this disruption. A 90° elbow forces the entire airstream through a tight right angle. The air has nowhere to go gradually — it slams into the outer wall, compresses, and then expands again on the far side of the bend, creating a chaotic pocket of pressure that the motor has to work against.

Two 45° bends covering the same directional change allow the airstream to curve more gently. Although the total turning is identical (45° + 45° = 90°), the airflow never encounters a single abrupt wall. It redirects progressively, keeping the boundary layer attached to the duct wall for longer and losing far less energy in the process.

The role of duct diameter

Diameter compounds the effect. Narrower ducting means higher air velocity for the same volume, which increases the energy lost at every bend. This is why CATA recommends using the widest practical duct diameter for your installation — typically 150mm for most chimney and integrated hoods — and never reducing diameter to fit awkward spaces if it can be avoided. You can read more about choosing the right duct size in our guide to what size ducting to use for your cooker hood.

90° vs 45° — A Direct Comparison

The table below compares the key performance characteristics of both bend types across a typical domestic installation scenario.

Factor	90° Bend	Two 45° Bends
Directional change achieved	90°	90° (combined)
Typical pressure drop (150mm duct)	High — equiv. ~1.5m straight	Low — equiv. ~0.9m combined
Turbulence created	Significant — recirculation zone at outer wall	Minimal — airflow stays attached
Airflow loss (short 2m run)	Up to 50%	15–25%
Noise impact	Increases turbulent noise, especially at higher speeds	Noticeably quieter at equivalent speeds
Space required	Compact — fits tight corners	Needs slightly more linear space for both fittings
Best used when	Space constraints leave no alternative	Wherever geometry allows even slight planning

Performance at a glance

To illustrate the airflow impact in a typical 2-metre duct run, the comparisons below reflect relative extraction performance relative to the hood’s rated output.

No bends (baseline)~90% rated output

Two 45° bends~75% rated output

One 90° bend~55% rated output

Two 90° bends~35% rated output

Approximate figures for 150mm rigid smooth ducting. Results vary with duct length, hood power, and duct material.

Equivalent Duct Length: The Industry Calculation

The building services and ventilation industries use a concept called equivalent lengthEquivalent length converts the resistance of fittings (bends, offsets, terminal grilles) into metres of straight duct with the same pressure drop. This allows installers to assess total system resistance in a single figure. to account for the resistance that bends add to a duct system. Rather than calculating complex fluid dynamics, you convert each fitting into an additional number of equivalent straight metres, add them to your actual duct length, and check the total against your hood’s performance curve.

For 150mm rigid circular ducting, commonly used equivalent lengths are:

90° elbow: approximately 1.5m equivalent
45° elbow: approximately 0.6m equivalent (so two 45° bends = ~1.2m)
Flat offset (two 45° bends pre-assembled): approximately 1.0–1.2m equivalent
Wall terminal / louvre grille: approximately 1.5m equivalent
In-line duct fan: negligible additional resistance (it adds power rather than resistance)

If your physical duct run is 2 metres of straight pipe and you need one 90° bend and a wall terminal, your effective total is approximately 2 + 1.5 + 1.5 = 5m equivalent. That is a very different picture from the raw 2 metres — and it explains why so many installed hoods fail to meet their rated performance. The UK Government’s Approved Document F covers ventilation standards for dwellings and provides useful context on acceptable system resistance for domestic extraction.

Airflow Impact Calculator

Estimate Your Duct System’s Effective Airflow

Enter your hood’s rated airflow and duct configuration to see the estimated real-world output after resistance losses.

Hood rated airflow (m³/h)

Straight duct length (metres)

Number of 90° bends

Number of 45° bends

Wall/roof terminal?

Duct material

Hood rated output—

Straight duct equivalent—

Bends equivalent—

Terminal equivalent—

Total effective duct length—

Estimated effective airflow—

When a 90° Bend Is Unavoidable

Real kitchens rarely offer the luxury of ideal duct routing. Structural beams, existing pipe runs, awkward wall positions, and compact integrated hood designs often make a 90° bend inevitable. In these situations, the goal shifts from elimination to mitigation.

Common mistakes that compound 90° bend losses

Positioning a 90° bend immediately at the hood outlet, where air velocity is at its highest
Using flexible corrugated ducting around bends — the accordion surface dramatically amplifies resistance
Reducing duct diameter before a bend to fit a smaller elbow
Installing two 90° bends back to back without a straight run between them
Using an undersized wall terminal that constricts airflow at the final exit point

How to minimise the impact

If you are working with one unavoidable 90° bend, a few practical choices can limit the damage. Position the bend as far along the duct run as possible, away from the hood outlet, so the highest-velocity air has time to slow and stabilise before hitting the change in direction. Use a swept elbow rather than a sharp mitre-cut fitting — a swept elbow has a larger radius and produces meaningfully less turbulence. Keep both the duct section before and after the bend as straight and as long as practically possible. And ensure you compensate by specifying a hood with sufficient headroom in its rated airflow to absorb the efficiency loss.

Rule of thumb: For every 90° bend in your duct run, add 1.5m to your effective duct length calculation. If your total effective length exceeds 5m, consider upgrading to a more powerful hood or adding a supplementary in-line duct fan to restore extraction performance.

Planning Your Duct Run for Maximum Performance

The best time to address ducting bends is before installation begins. Once a hood is fitted and ducting is run through finished cabinetry or walls, making changes is expensive and disruptive. A few minutes spent with a pencil and the steps below can prevent performance problems that persist for the life of the appliance.

1
Map every direction change Sketch the planned duct route from hood outlet to external wall or roof. Mark every point where the duct changes direction, even gentle ones. This gives you a complete picture of bend count before committing to a layout.
2
Calculate equivalent length Add up your straight duct length plus the equivalent metres for each fitting. Use the figures above: 1.5m per 90° bend, 0.6m per 45° bend, 1.5m for the wall terminal. The total should ideally stay under 5m equivalent for most domestic hoods.
3
Replace 90° bends with 45° pairs where possible On paper, this is a straightforward swap. In practice, it may require shifting the outlet position slightly or adjusting cabinetry, but even a single substitution delivers a measurable improvement in airflow and noise.
4
Choose rigid smooth ducting throughout Flexible corrugated duct is convenient but costly in performance terms. Use rigid round or flat-channel duct for the bulk of the run and limit flexible sections to short, straight connections at the hood outlet only.
5
Verify your hood’s rated output allows for the planned resistance If your equivalent length exceeds 5m, check that the hood’s rated m³/h at higher resistance settings — sometimes listed in the product datasheet — still meets the ventilation requirement for your kitchen. A hood rated at 600 m³/h under ideal conditions may only deliver 300 m³/h through a demanding duct run. Our guide to cooker hoods and kitchen condensation explores why adequate extraction rates matter beyond just cooking odours.

Frequently Asked Questions

Yes, significantly. Rigid smooth plastic or aluminium ducting produces the least resistance at any bend. Semi-rigid corrugated ducting adds friction across its entire surface, which compounds the loss at each bend. Fully flexible corrugated spiral duct is the worst performer — its concertinaed interior creates turbulence even in straight sections, and around any bend this becomes substantially worse. Where flexible duct is unavoidable, keep the section as short and as straight as possible, and never allow it to kink or compress.

In theory, no — the fluid dynamics consistently favour the gentler change in direction. In practice, the only scenario where two 45° bends could underperform is if they are installed very close together with no straight run between them, forcing the airstream through two rapid directional changes almost simultaneously. Ideally, leave at least 150mm (one duct diameter) of straight duct between any two fittings to allow the airflow to stabilise before the next change in direction.

The pressure drop is the same regardless of orientation — the resistance is caused by the change in direction of the airstream, not by gravity. However, vertical bends (turning air upward) can accumulate condensation at the outer wall of the elbow more readily. This is rarely a performance issue in properly insulated ductwork, but it can cause noise from dripping and, in poorly sealed runs, moisture damage over time. Ensure any vertical sections slope towards the external exit where possible.

There is no fixed regulatory maximum for the number of bends in domestic cooker hood ducting, but UK Approved Document F (ventilation standards) requires that extraction systems meet minimum performance thresholds. Most hood manufacturers recommend a maximum equivalent duct length of 3–6 metres depending on the model — any duct run exceeding this, including the contribution of all bends, risks failing to meet the ventilation rate required for your kitchen size. As a practical guide, try to keep the total bend count to two or fewer, and calculate your equivalent length to verify compliance before installation.

To a degree, yes. A hood with a higher rated airflow will still move more air through a resistive duct run than a lower-rated model in identical conditions. However, this is an expensive workaround with a noise penalty — a high-powered motor pushing through a restrictive duct will be significantly louder, often negating one of the key reasons for specifying a premium hood in the first place. The better approach is to fix the ductwork rather than overspecify the appliance.

Flat-channel (rectangular) ducting bends produce slightly different pressure drop characteristics because the airflow cross-section is non-uniform around the turn. In general, flat-channel bends are comparable to round duct bends at the same nominal equivalent diameter, but the fittings must be specifically designed for flat channel — using improvised transitions or undersized flat-channel bends will worsen performance. Where flat channel is used for aesthetic reasons through cabinetry, always ensure the transition back to round duct (at the hood outlet and at the external wall) uses properly matched adaptor pieces.

Key Takeaways

A single 90° bend can reduce cooker hood airflow by up to 50% — far more than most people account for during installation planning.
Two 45° bends achieve the same 90° direction change with roughly half the pressure drop of a single sharp elbow.
Use equivalent duct length calculations (1.5m per 90° bend, 0.6m per 45° bend, 1.5m for the wall terminal) to understand your system’s true resistance before specifying a hood.
Rigid smooth ducting consistently outperforms flexible corrugated alternatives, especially around bends.
Position any unavoidable 90° bends as far from the hood outlet as possible, and use swept elbows rather than sharp mitre fittings.
If your total equivalent duct length exceeds 5m, upgrade the hood’s rated airflow or add an in-line duct fan rather than accepting the performance loss.

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