I wish I could just tell you one exact measurement, but not only do different indoor plants have different needs for light (no shocking news there), but their needs also change depending on what growth stage they're in. Plus the intensity of the grow light also changes how close or far away they should be positioned.
So coming up below I've included the distances and considerations for germinating seeds, for growing seedlings, for unrooted vs rooted cuttings, for mature plants needs for growing foliage vs flowering; as well as what area size different lights cover, and the best-match options for different situations in commonly available lights.
Let's jump in and shed some light on the question (sorry, couldn't resist the pun), of the right distance to position grow lights from your indoor plants...
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The 'General Rule' when lighting indoor plants
As a general rule-of-thumb for choosing LED grow lights, indoor plants being grown for their foliage need 10 to 40 watts of light per square foot.
Go for the lower end for indoor plants that need low to medium light levels, or for plants that already get some natural light and just need a boost. Or look at the middle to higher end of that range for houseplants needing very bright indirect or direct-sun.
But unless your plants needs a few hours direct sun daily, I'd aim for around 20 to 30 watts maximum for most indoor plants.
If you've got a plant that's usually grown outdoors and needs direct sun, have a look at options over 30 watts (like the Sansi 36 watt).
Lighting up a small plant shelf
So that you can do your own measurements, one square foot is 30cms x 30 cms (about 12 x 12 inches).
A small to medium size plant shelf, or a collection of plants on a coffee table, might be around two square foot for example.
That means a light like the clip-on PHOTO+ Grow Lights - which have both 20 watt and 30 watt options - or a Sansi 24 watt put in a desk or floor lamp - could all be your perfect solution for lighting up a small'ish plant shelf.
One reason this only a general rule however, is because wattage and brightness aren't everything that matters.
A lower wattage light might be less bright to our eyes, but produce better quality of light for our plants.
So definitely take the time to read up about the light you're looking at buying. Most suppliers provide a LOT of information to help you work out if it's the right light for your needs.
(or for those in New Zealand you're welcome to ask me, I'm happy to help you choose the right light)
A warning before you buy grow lights
Before you buy grow lights for your indoor plants based on the 10 to 40 watts 'golden rule' keep in mind most indoor plants get some light from natural sources also. It's rare your grow light will be your only source of light in an otherwise dark room.
That means your grow light does NOT have to meet your plant's needs all by itself (and if it did, you might be going overboard which can result in bleached and scorched leaves, ouch!).
One lower wattage grow light may be plenty when there's good natural light for at least some of the day.
Like the PHOTO+ double (which is 20 watts) for example, or one or two 15 watt Sansi grow bulbs (Sansi are also available here on Amazon for those not in New Zealand).
Watch out for 'tomatoes'
Also keep in mind a lot of indoor plant grow light advice online is NOT about what you and I call houseplants. It's based on growing, um, tomatoes (if you know what I mean).
'Tomatoes' do not have the same bright indirect light requirements that the majority of our Amazon rainforest floor and rainforest canopy plants do. 'Tomatoes' need more like 30 to 50 watts per square foot when using LED grow lights - and to flower, it's more like 40 to 60 watts per square foot.
If your source doesn't outright say it's for 'tomatoes' then look for what the website sells (if it's a hydroponics store or brand, that's a common giveaway).
Buying a grow light for your Monstera, Philodendrons, Pothos and similar, based on advice for 'tomatoes', could mean you end up burning and bleaching your plants.
How close should grow lights be for cuttings and for germinating seeds?
For germinating most seeds and for unrooted cuttings, LED grow lights can be further away or dimmer, as light requirements are lower at this early stage.
Start around 60cms away, or if your grow light has adjustable brightness, turn the brightness down and position them closer, around 30cms.
What distance should grow lights be for seedlings and rooted cuttings?
Once seeds have germinated and started growing their first true leaves, or when rooted cuttings and recently potted up cuttings begin producing new foliage, move your grow lights closer or turn the brightness up, as the light requirement goes up considerably at this stage.
Depending on the heat and intensity of your chosen grow light, you could go as close as 15cms, but as a general rule start around 30cms and adjust closer (or further away), if needed.
A cool-running LED grow light is a good idea for this stage too as tender new foliage can be more heat sensitive. Keep an eye out for leggy, stretching stems or smaller leaves (a sign more light may be needed and grow lights could be turned up or moved closer). Stunted or very compact growth can be a sign the light is too intense.
Also watch for fading or bleached leaf colour or scorching. Also a sign to turn the brightness down or shift the lights further away.
For my rooted cuttings and baby plants in my converted Ikea cabinet (I have a Rudsta Wide, below), I use the strip-type LED grow lights.
For the locals (if you're based in New Zealand), the ones I use in my Ikea cabinet are these Grow Bars, or for those not in New Zealand, have a look at the Barrina T5's on Amazon.
For my cuttings and younger plants 'out in the open' I just clip one of the PHOTO+ type lights to the bench, shelf or table that they're on. I find those run really cool also. If you go for the clip-on type, I'd suggest getting at least a double and positioning the two tubes either side for good coverage, so plants don't lean towards a single light source. Singles are great value but better only as a supplementary light source in an otherwise well lit area, since they only offer light from one direction.
If you don't have anything to stick-on or clip-on your lights to, but just need a light boost for dark mornings or evenings, a Sansi 10 watt is a great option instead. They're not much bigger than a standard light bulb and fit in most standard desk lamps and floor lamps.
What distance should LED grow lights be for established indoor plants?
Once your plants have matured a bit, or if you've bought them from your local garden centre already well established, they still have a higher light requirement if your focus is on growing plants for their foliage, not flowers.
This is where the range of 20 to 30 watts per square foot is helpful to know (lower for light sensitive, and higher for direct sun lovers), taking natural light into account also.
My largest plant shelf is so big (about 2 metres wide x 1.8 metres tall which is approx 80 inches x 70 inches), that there aren't many places it fits at home.
The natural light level where I have it is low to medium light at best, making winter a struggle, which means grow lights are a must for me.
I use the Sansi grow light bulbs in floor lamps for them. The Sansi ones are very bright and intense so I position them further away. I use two Sansi 24 watt bulbs to cover my largest plant shelf, positioned about 1.5 to 2 metres away from my plants, either side of my plant shelf so plants don't lean or get leggy (for my international plant friends, you can get the Sansi 24 watt on Amazon too).
But unless you've got an extra-large area to cover, two Sansi 15 watt bulbs are usually plenty for most size plant shelves and small plant rooms.
Most of my smaller plant shelves are lit by stick-on strip-type grow lights (I use Grow Bars myself but if you're not in NZ a similar type are Barrina T5's available on Amazon), which I stick on above the middle and lower shelves where natural light is naturally lower. They are the same ones I use in my Ikea cabinet.
For lower wattage solutons like those Grow Bars and the double and triple PHOTO+ (which are 10 watts per light tube), I position them anywhere from 15cms to 30cms away from my established plants.
It's still best to start around 30cms away and adjust from there based on how your foliage handles the light.
If you're worried about your sensitive or variegated foliage, look for lights where you can either turn the brightness down, or they mention they're made for sensitive or variegated foliage (like the PHOTO+ are).
What distance should LED grow lights be for flowering indoor plants?
For flowering, some plants need the light level lowered, some increased, depending on variables such as whether they are summer or winter flowering.
If your plant isn't flowering, look up first if 'not enough light' is a common cause of not flowering, which is the case for a lot of our indoor plants such as Peace Lilies, Orchids, Anthurium, Hoyas and African Violets; and for those guys, increase their light level.
That doesn't mean you need to go out and buy a new grow light. You can also simply shift your grow light closer, have it on for more hours, or turn the brightness up.
Keep learning about grow lights
Yes, this is a VERY simplified way of working out the general distance away from plants that your grow light should be for different plants at different stages of growth.
There is WAY more to the complex world of grow lights. Wattage may a common way to help select grow lights, but really isn't the ideal deciding factor by itself, especially if your plants are relying on your grow lights for their only source of light.
If you want to learn more about the different terms and find out more answers to grow light FAQs, including what lux, PAR, IP, Kelvin and more mean (and why they matter), this one's for you...
Read the ultimate guide to choosing grow lights for indoor plants >