How Many Lumens Do I Need?

Getting just the right amount of light in a room is critical to not only making it habitable, but enjoyable and adaptable for different times of the day. Not only will you need enough light to see all that you need to, but certain spaces require specific foot candle or lumen levels (the units used for measuring how much light falls onto an area), to light your place well.

Whether this is for your home or office, let us show you how to calculate how many lumens you need to light your room and make a space well lit.

Common Terms You Need To Know

Lumens

Lumens are the metric measurement of light that comes from a source. The more lumens, the brighter it is. For example a 100 watt incandescent bulb produces about 1,500 lumens.

Foot Candles

Foot candles are an imperial unit of measurement used in the USA to calculate how much or how little light falls onto a certain area. One foot candle is roughly the equivalent of one lumen per square foot.

Lux

Lux is a metric unit that measures the intensity of light within a space with its source being lumens per square meter. Lux is the most accurate unit of measurement because it takes into account how small (or large) an area actually is and adjusts accordingly. One lux equals one lumen per square meter or 10.7664 foot candles.

Watts

Watts is the measurement of how much power is used to create the light, not the level of brightness as it used to known for back when incandescent lighting was the primary option. In the age of LED the Watt as a unit is not to be used to compare light output however tempting it may be!

Watts to Lumens Comparison Chart

Different light technologies convert power to light in varying efficiencies, giving them a different lumen-per-watt ratio.

Here is a chart for how many watts equals how many lumens according to the type of light source. Lumens can vary in different light bulbs and products even if the same technology.

Efficacy

The efficiency of an LED source is largely dependent on how well each product is made and the type of LED chip they use. LED technology is still getting more efficient over time as manufacturers keep pushing for more light for less power.

Light Output Ratio (LOR)

LOR stands for Light Output Ratio. Designers also consider the LOR of a product as it defines how much light is lost in a lighting fixture and therefore help quantify exactly how much of the lumens being created inside the product are actually being delivered outside of the product and into the room.

LOR is calculated by dividing the total light output of the lighting fixture by the light flux or lumens inside the lamp or product. This ratio can look like 70 LOR on packaging, meaning you will actually be getting 70% of the stated lumen level.

Lumens per Square Foot Calculation

The imperial calculation still used in the USA today:

1. Calculate The Room’s Square Footage

Multiply the Length of a Room by the Width to find out how many square feet it is. For example, if a room is 20 feet long and 10 feet wide, it would have 200 square feet.

2. Look Up the Recommended Foot Candles by Room Type or Function

In the USA you start with using foot-candles to get to the required lumen level. Rooms need different amounts of light. For example, a bathroom or kitchen needs more foot-candles than a living room or bedroom.

3. Convert to Lumens

The amount of light in a room is measured in lumens. To find out how many lumens are needed, multiply the square footage of the room, by the foot-candles you need.

So for example, if you have a 200 square foot living room which needs 10-20 foot-candles, it will need 2,000-4,000 lumens.

If you have a 200 square foot dining room which needs 30-40 foot candles it will need 6,000-8,000 lumens.

Lumens per Square Metre Calculation

The metric lumen calculation used in worldwide:

1. Calculate The Room’s Square Meterage

Multiply the Length of a Room by the Width to find out how many square metres it is. For example, if a room is 10 metres long and 5 metres wide, it would have 50 square metres.

2. Look Up the Recommended Lux Level by Room Type or Function

As Rooms need different amounts of light, use the required lux level for it’s purpose to give a general quantity of light required. For example, a home office will need more light than a lounge.

3. Calculate Lumen Level

The amount of light in a room is measured in lumens. To find out how many lumens are needed, multiply the square meterage of the room, by the lux level you need. So if you have a 50 square metre living room which needs 150 lx p/m, the total amount of lumens required will be 7500 lm.

How Many Lumens?

These calculations give a broad understanding of the overall amount of light required in that room for it’s intended use.

However, good lighting design then goes beyond this base level of knowledge and works into the areas that will need specific lighting levels.

Here’s an example of how within a space there are specific lumen levels required to suit activities:

1. Calculate the Overall Lumen Level Required

An average space of 250 square feet, you will need 5,000 lumens as your primary light source.

This would be 20 lumens for each square foot throughout the room.

2. Calculate Specific Lumen Levels for Tasks within the Space

However, where you eat will need 30 lumens per square foot so if your have a dining room table is 6 x 3 feet (18 sq ft), then it would take 18 x 30lm = 540 lumens to light the table adequately.

3. Consider Other Influencing Factors

Lighting design also considers reflectance, just how well the light will travel around the room.

Roughly speaking, a darker colored room with deep colored fabrics or walls and floors, will need compensating for by adding roughly 10 lumens per square foot to the calculations.

How to Light a Space

Once there is sufficient light within a room, further choices become subjective to the occupant.

You may prefer more light on during the evening than your partner for instance. One of you could have eyesight that is weaker too? For flexibility always increase the lumen levels by 10% to 20% which should satisfy everyone’s needs and with a dimmer installed the brightness can be adjusted to suit personal taste.

Great lighting design goes even further and uses the contrast between each specifically lit area to weave together highs and lows of light intensity that when combined become unified and create a wonderfully lit room.

Andrew Orange, the owner of Orange Lighting qualified and worked as an interior designer in 1993 before specialising in lighting working on high profile projects based in London. Since starting Orange Lighting Ltd in 2003 he has been sharing his knowledge and unique teaching style mostly to his designer clients, offering practical real life advice born from running a busy consultancy and lighting supply business. Launching in 2020, his blog has evolved into Quick & Easy Lighting, curating some 25 years design experience into making the lighting choice and design process achievable and easy to understand for all.