How many BTU do I need for a 12 m² (12×12 ft) room in Sri Lanka?

A 12 m² room (about 130 sq ft) has a base load of 12 × 600 = 7,200 BTU/h. For a shaded, standard-ceiling bedroom with two people that rounds up to a 9,000 BTU (1.0 ton) unit; a sunny or west-facing room of the same size pushes toward 12,000 BTU. Enter your exact conditions above for the precise figure.

Is a 1 ton (12,000 BTU) AC enough for a bedroom?

Usually yes. A 12,000 BTU unit comfortably cools a normal Sri Lankan bedroom up to about 18–20 m² (roughly 200 sq ft) with a standard ceiling and two occupants. Larger, sunnier, or high-ceiling rooms, or ones with many people, need 18,000 BTU (1.5 ton) or more.

How much does it cost to run an air conditioner per month in Sri Lanka?

It depends on size, hours, and your CEB rate. A 12,000 BTU inverter running 8 hours a day draws about 1.1 kW, using roughly 260 kWh a month. At an upper-block marginal rate of Rs 45/kWh that is about Rs 11,800; at Rs 60/kWh it is closer to Rs 15,700. The calculator shows your figure and links to the CEB bill calculator for a block-exact total.

What is the difference between inverter and non-inverter running cost?

An inverter compressor varies its speed and runs at a higher average efficiency (EER around 11 versus about 9.5 for a fixed-speed non-inverter). For the same cooling, an inverter uses roughly 13–15% less electricity, so on a unit running several hours a day it typically saves Rs 1,000–3,000 a month depending on size and tariff.

What happens if my AC is undersized or oversized?

An undersized unit runs non-stop and never reaches the set temperature, wearing out faster. An oversized unit cools the air quickly then switches off before it has removed humidity, so the room feels cold and clammy; this short-cycling also raises wear and the bill. Matching capacity to the actual load — what this tool does — avoids both.

Why does this use a higher BTU figure than US or UK calculators?

Most online calculators assume a temperate climate at roughly 20 BTU per square foot. Sri Lanka's sustained ~30 °C heat and high humidity add a large latent (moisture-removal) load, so local practice sizes closer to 55–60 BTU per square foot (600 BTU/m²). Using a temperate chart here would leave you with an AC that struggles on a hot afternoon.

Does the calculator account for windows, insulation, and appliances?

It uses the recognised area-plus-adjustments rule of thumb (sun exposure, ceiling height, occupancy, kitchen heat) rather than a full Manual-J survey of every window and wall. That is accurate enough to pick the right standard unit for a home or small office. For a data centre, glass-walled room, or large commercial space, get an HVAC engineer's load calculation.

What size AC is right for a living room versus a bedroom?

Living rooms are usually larger, have more people, higher ceilings, and more sun, so they need more capacity than a bedroom of the same floor area. A typical Sri Lankan living room of 20–28 m² lands on 18,000–24,000 BTU (1.5–2.0 ton), while a bedroom of 10–16 m² is usually 9,000–12,000 BTU (1.0–1.5 ton).

When was this calculator last verified?

The sizing rules were cross-checked against ENERGY STAR room-AC guidance and the running-cost method against the CEB/PUCSL domestic tariff on 2026-07-11. Rates are reviewed after any PUCSL tariff revision.

Sri Lanka · Utility

Air Conditioner Size (BTU) Calculator for Sri Lanka

Work out the correct AC capacity in BTU and tons for any room, tuned for Sri Lanka's hot-humid climate — then see the estimated monthly running cost on the CEB tariff, inverter versus non-inverter. No signup, sources cited below.

By Induwara Ashinsana— Executive Director, Ryzera TechnologiesUpdated Jul 11, 2026

What size AC do you need?

ENERGY STAR · CEB method

Quick start

Room floor area

m²

Length × width of the room. A 12 m² bedroom is roughly 3.5 m × 3.5 m.

Ceiling height

Sun exposure

Room type

Regular occupants

People usually in the room together.

AC type

Usage (hours/day)

For the monthly running-cost estimate.

Electricity rate (Rs/kWh)

CEB upper-block marginal ≈ Rs 45–75.

Recommended size

9,000 BTU

0.75 ton

Cooling load needed

7,200 BTU

20% spare capacity

Power draw (inverter)

0.82 kW

EER 11 · 196 kWh/mo

Est. running cost / month

Rs 8,836

Saves ~Rs 1,395/mo vs non-inverter

A 9,000 BTU (0.75 ton) unit fits this room with 20% spare. Avoid going far larger — an oversized AC short-cycles, cools without removing humidity, and costs more to run.

Load & cost breakdown

Step	Value
Base load (area × 600 BTU/m²)	7,200 BTU
Ceiling factor	×1.00
Sun factor	×1.00
Extra occupants (600 BTU each over 2)	+0 BTU
Kitchen heat	+0 BTU
Required cooling load	7,200 BTU
Recommended standard unit	9,000 BTU(0.75 ton)

Running cost / month	Power	kWh/mo	Cost
InverterEER 11	0.82 kW	196	Rs 8,836
Non-inverterEER 9.5	0.95 kW	227	Rs 10,232

Power draw cross-checked two ways: BTU/h ÷ EER and the SI/COP conversion — both agree to under a watt. Costs assume steady-state running for the hours you set; real inverter bills are usually a little lower because the compressor throttles down once the room reaches setpoint.

Sizing method: ENERGY STAR room-AC area rule with an elevated tropical base load (ASHRAE-justified for hot-humid climates). Running cost uses manufacturer EER conventions (inverter 11, non-inverter 9.5 BTU/h per watt) and a flat CEB/PUCSL marginal rate. For a block-accurate bill, use the CEB electricity bill calculator linked below. This is a rule-of-thumb estimate, not a Manual-J audit.

How it works

Air conditioners are rated by cooling capacity in BTU/h (British Thermal Units per hour); 12,000 BTU/h is one refrigeration “ton”. Getting the size right matters because both directions cost money: an undersized unit never reaches the set temperature, while an oversized one short-cycles, leaves the room humid, and pushes up the bill. This calculator uses the ENERGY STAR room-air-conditioner sizing method, adapted with a higher base load for Sri Lanka's tropical conditions.

Base load. Floor area (converted to m²) is multiplied by 600BTU/m². This tropical base sits well above the temperate ~215 BTU/m² that US/UK charts assume, because our sustained ~30 °C heat and high humidity add a large moisture-removal (latent) load on top of the sensible load — the reasoning set out in the ASHRAE Fundamentals handbook and matching local dealer practice.
Ceiling height.More air volume needs more cooling, so a high ceiling multiplies the base by 1.15 and a very high one by 1.25 (ENERGY STAR's note on ceilings above the standard 8 ft / 2.44 m).
Sun exposure. A heavily shaded room needs about 10% less and a very sunny or west-facing room about 10% more (×0.90 and ×1.10) — the ENERGY STAR shade/sun rule.
Occupancy.Two people are already in the base figure; each additional regular occupant adds 600 BTU/h of body heat.
Kitchen heat.If the room is a kitchen, 4,000 BTU/h is added for cooking appliances, per ENERGY STAR.
Recommended unit. The required load is rounded up to the smallest standard capacity sold locally — 9,000 / 12,000 / 18,000 / 24,000 BTU. Loads above 24,000 BTU are flagged for multiple units or a ducted/cassette system.

Running cost.Electrical power is the cooling capacity divided by the unit's Energy-Efficiency Ratio (EER, in BTU/h per watt): kW = BTU/h ÷ EER ÷ 1000. Inverter units run at a higher average EER (11) than fixed-speed non-inverter units (9.5). Monthly energy is power × hours/day × 30, and cost is that times your CEB rate. The tool cross-checks the power figure a second way — through the SI conversion (1 BTU/h = 0.29307 W) and COP — and the two agree to under a watt. For a block-exact bill, use the CEB electricity bill calculator linked below; the flat marginal rate here is a close estimate for a household already in the upper consumption blocks that running an AC implies.

Worked examples

12 m² shaded bedroom · inverter · 8 h/day · Rs 45/kWh

Base: 12 m² × 600 = 7,200 BTU
Ceiling ×1.00, sun (shaded) ×0.90 → 7,200 × 0.90 = 6,480 BTU
Occupancy add: max(0, 2−2) × 600 = 0; kitchen: 0
Required = 6,480 BTU → nearest standard up = 9,000 BTU (0.75 ton)
Power = 9,000 ÷ 11 ÷ 1000 = 0.82 kW
Month = 0.82 × 8 × 30 = 196 kWh → × Rs 45 ≈ Rs 8,836
Non-inverter (EER 9.5): 0.95 kW → 227 kWh → ≈ Rs 10,232 (about Rs 1,396 more)

24 m² sunny living room · high ceiling · 4 people · inverter · 6 h/day · Rs 45/kWh

Base: 24 m² × 600 = 14,400 BTU
Ceiling ×1.15, sun ×1.10 → 14,400 × 1.265 = 18,216 BTU
Occupancy add: (4−2) × 600 = 1,200 BTU; kitchen: 0
Required = 18,216 + 1,200 = 19,416 BTU → nearest standard up = 24,000 BTU (2.0 ton)
Power = 24,000 ÷ 11 ÷ 1000 = 2.18 kW
Month = 2.18 × 6 × 30 = 393 kWh → × Rs 45 ≈ Rs 17,673
Note: an 18,000 BTU unit would be marginally undersized, so the tool steps up one size.

20 m² office · normal · standard ceiling · 2 people (boundary case)

Base: 20 m² × 600 = 12,000 BTU
Ceiling ×1.00, sun ×1.00 → 12,000 BTU exactly
Occupancy add: 0; kitchen: 0
Required = 12,000 BTU → matches the 12,000 BTU standard exactly (1.0 ton)
Zero spare capacity, so the tool flags 'tight fit — consider one size up in extreme heat'.

Frequently asked questions

Sources & references

Sizing rules were last cross-checked against ENERGY STAR guidance, and the running-cost rate against the CEB/PUCSL domestic tariff, on 2026-07-11. The page is reviewed after any PUCSL tariff revision.

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