# AC Tonnage Calculator: Screening the Cooling Load of an Office

> AC tonnage is the rate at which an air-conditioning system removes heat, measured in tons of refrigeration (TR); one TR is 12,000 BTU/h or about 3.5 kW. A screening calculation adds up the heat from people, fresh air, lighting, equipment and the building envelope to give a first estimate of the tonnage an office needs.

## What it calculates

The calculator gives a **screening estimate of the cooling load** for an office floor or
zone: the total heat gain in BTU/h, the equivalent tons of refrigeration (TR) and kilowatts
of cooling, and the floor area each ton serves. It also breaks the load down by source so
you can see what is driving it.

It is designed for the stage before an HVAC engineer is involved: comparing buildings,
checking whether a landlord's installed capacity is in the right range, or deciding how much
plant space and budget to allow.

## Inputs

**Conditioned carpet area** is the floor area the system cools. Use carpet area, not the
chargeable area on the lease.

**Peak occupants** is the number of people in the space at the busiest time.

**Exposure to sun and outside air** chooses an envelope allowance: low for an interior or
shaded floor with little glass, average for a typical intermediate floor with some windows,
and high for large west or south glazing or a top floor under the roof.

**Lighting load** is in watts for each square foot of floor. If you have used the
lighting calculator, divide its connected load by the floor area.

The **assumptions** panel holds the envelope allowance itself, the equipment heat for each
person, the heat from each person, the fresh-air load for each person and the margin.

## Method

Each heat gain is worked out in BTU/h and added up:

- **People** = occupants × heat from each person.
- **Fresh air** = occupants × fresh-air load for each person.
- **Lighting** = area × lighting load × 3.412.
- **Equipment** = occupants × equipment watts × 3.412.
- **Envelope** = area × envelope allowance.

The total is increased by the margin, then divided by 12,000 to give tons of refrigeration.
One watt is 3.412 BTU/h, and one TR is 12,000 BTU/h or 3.517 kW.

## Assumptions

| Assumption | Default | Basis |
| --- | --- | --- |
| Heat from each person | 450 BTU/h | Sensible plus latent heat for moderately active office work, close to the value ASHRAE tabulates |
| Fresh-air load for each person | 1,000 BTU/h | About 15 cfm of outside air for each person, cooled and dried from humid outdoor conditions |
| Equipment for each person | 150 W | Laptop, monitor and a share of printers and pantry equipment |
| Lighting | 0.9 W for each sq ft | About 9.7 W/m², a typical LED office |
| Envelope, low / average / high | 20 / 30 / 40 BTU/h for each sq ft | Screening allowance for heat through walls, glass and roof |
| Margin | 10% | Allowance for items not listed |

The envelope allowance is the roughest of these. Heat through glass depends on orientation,
shading, glass type and the hour of the day, and a heat-load calculation treats each
façade separately. The three presets are deliberately broad; change the figure if you know
the building is better or worse.

The ventilation rate behind the fresh-air figure is set by the project's HVAC design under
ASHRAE 62.1 or NBC 2016 Part 8. A different rate or outdoor condition changes this load
directly.

## Limitations

- **It is a single peak figure.** A heat-load calculation checks the load hour by hour,
  because the peak on the west façade comes in the late afternoon while the peak on the
  east comes in the morning.
- **It does not split sensible and latent load.** Equipment selection needs both, because
  dehumidification is a separate duty from cooling.
- **It does not size zones.** Cabins, meeting rooms and server rooms have very different
  loads and need their own units or zones.
- **It ignores special rooms.** Server rooms, kitchens and labs need their own calculation.
- **It does not choose equipment.** The choice between split units, VRF systems and chilled
  water depends on the building, the load profile and the landlord's infrastructure.

## Worked example

A 5,000 sq ft office with 50 people on an intermediate floor with some windows, using the
default assumptions:

| Heat gain | Calculation | BTU/h |
| --- | --- | --- |
| People | 50 × 450 | 22,500 |
| Fresh air | 50 × 1,000 | 50,000 |
| Lighting | 5,000 × 0.9 × 3.412 | 15,354 |
| Equipment | 50 × 150 × 3.412 | 25,590 |
| Envelope | 5,000 × 30 | 150,000 |
| Subtotal | | 263,444 |
| Margin, 10% | | 26,344 |
| Total | | 289,788 |

The total is 289,788 / 12,000 = **24.1 TR**, or about 85 kW of cooling, with each ton
serving about 207 sq ft. Fresh air is about 19% of the load before margin.

Double the occupancy to 100 people and the load rises to about 33 TR, around 151 sq ft for
each ton, without any change to the building. That is why a single area-per-ton rule of
thumb can be badly wrong in either direction.

## How engineers use it

An HVAC engineer starts from the same heat gains, but calculates each one properly: the
envelope from the actual wall and glass construction, orientation and shading; the
occupancy and equipment from the layout; the fresh air from the ventilation design and the
local design weather. The calculation is run for each zone and for each hour, and the
system is sized on the coincident peak.

A screening figure like this one is still useful to an engineer early on: it shows whether
the landlord's existing capacity is likely to be enough, how much space the plant will need
and what order of budget to allow, before the layout is fixed.

## When a professional design must replace it

Use the calculator to compare options, to check a landlord's installed capacity and to set
an early budget allowance. Replace it with a heat-load calculation by an HVAC engineer
before any equipment is selected or ordered, before the ductwork or piping is designed, and
whenever the building has unusual glazing, occupancy or process loads. Where the project
must comply with ECBC 2017, the engineer also checks the equipment efficiency and controls
against its requirements.

## Frequently asked questions

### How many square feet does one ton of AC cover in an office?

There is no fixed answer, which is why this calculator works from the heat gains instead. A sparsely occupied interior floor might need one ton for every 250 sq ft or more, while a densely occupied top floor with west-facing glass might need one for every 130 sq ft or less. Occupancy, fresh air and sun exposure make the difference.

### Why does fresh air add so much load?

Outside air brought in for ventilation has to be cooled and, for much of the year in India, dried as well. Removing moisture from humid monsoon air takes a lot of energy. In a well-occupied office, fresh air can account for a fifth or more of the total cooling load, which is why ventilation rates and heat recovery matter.

### Should I size the AC on carpet area or chargeable area?

On the area you actually condition, which is your carpet area. Chargeable area includes lobbies, lift cores and staircases that your system does not cool. Sizing on chargeable area oversizes the plant, which costs more to buy and runs less efficiently.

### What is the difference between TR and kW?

Both measure cooling capacity: one TR equals about 3.517 kW of cooling. Neither is the electricity the system uses. The electrical input depends on the efficiency of the equipment, which is a separate figure on the datasheet.

### Can I buy equipment based on this result?

No. The result is a screening estimate for budgeting and space planning. Equipment selection needs a heat-load calculation by an HVAC engineer, done for the real building, orientation, glazing and design weather data, and checked at the hours of peak load.

## Sources

- [ASHRAE Handbook — Fundamentals](https://www.ashrae.org/technical-resources/standards-and-guidelines) — Nonresidential cooling and heating load calculations
- [ASHRAE 62.1](https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2) — Ventilation for acceptable indoor air quality
- [NBC 2016, Part 8, Section 3](https://www.bis.gov.in/standards/national-building-code/?lang=en) — Air conditioning, heating and mechanical ventilation
- [ECBC 2017](https://beeindia.gov.in/) — Comfort systems and controls

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Source: https://hagerstone.com/calculators/ac-tonnage-calculator