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    Martha tent calculator

    A Martha tent shelf holds about four 5-lb fruiting blocks, so the tent you need follows from how many blocks you fruit at once. This calculator sizes the tent, estimates the mist your fan's fresh air calls for, and adds up power and water for a month.

    Martha tent calculator

    1. What size tent?

    About four fit on a shelf.

    Where does the exhaust fan sit?

    Tent to get

    4-tier tent

    • Holds about 16 five-pound blocks
    • Your blocks fill 4 shelves
    • About 500 L of air; most tents share a 27 × 19 in footprint

    2. How much mist does your fan's fresh air need?

    CFM

    At the speed you run it, not its top rating.

    min

    60 is around the clock; GroCycle starts at 2.

    °F
    % RH
    % RH

    85–95% while fruiting.

    Mist needed

    732 mL/hr

    • About 17.6 L of water a day
    • A rough estimate: compare it with your humidifier's rated output
    • About 150–250 mL/hr suits a 4-tier tent

    3. What does it cost to run?

    W
    hr
    mL/hr

    Its rated mist, for the water it uses.

    W
    hr
    W

    Counted around the clock.

    ¢/kWh

    Electricity a month

    $4.61

    • 0.84 kWh a day
    • Water: about 2.9 L a day, 23 gallons a month

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    How does the martha tent calculator work?

    • Blocks a tent holds = shelves × 4, less one shelf if the fan sits on the top one
    • Mist (mL/hr) ≈ fan airflow × saturation vapor density × (tent RH − room RH)
    • Power per month = watts × hours a day × 30 ÷ 1,000 × your rate

    For example: A fan moving 50 CFM around the clock pulls in about 85 cubic meters of room air an hour. At 68°F a cubic meter of air holds about 17.3 g of water when saturated, so lifting that air from 40% to 90% RH takes 85 × 17.3 × 0.5, a little over 730 mL of mist an hour. Our Martha tent guide rounds that to about 700 mL/hr, several times the 150–250 mL/hr that suits a 4-tier tent. That is why most tents run the fan on a cycle.

    • The mist figure is a rough estimate. It assumes the tent sits at room temperature, and it leaves out the water the blocks give off and the mist that settles on the walls.
    • Tent volumes are approximate, from real products' listed sizes. Shelf load ratings vary by brand, so check yours before you load a shelf.
    • The power example uses the US average residential rate of 18.31 cents per kWh in July 2026 (EIA). Enter the rate on your own bill.

    Where do the numbers come from?

    Every formula and figure on this page is from one of our grower guides, which cite their sources: extension services, farm trials and published studies.

    Common questions

    How many blocks fit in a Martha tent?

    About four 5-lb blocks a shelf: 16 in a 4-tier tent and 20 in a 5-tier, or four fewer with the fan on the top shelf. North Spore rates each shelf of its tent at 20 lb or 4 fruiting blocks.

    How big a humidifier does a Martha tent need?

    About 150–250 mL/hr of mist suits a 4-tier tent. Fresh air raises the need, because every bit of room air the fan pulls in has to be brought up to fruiting humidity. Run the fan faster and you'll refill the humidifier more often.

    How often should the fan run?

    GroCycle starts at 2 minutes every hour, or 15 minutes every 4 hours if your timer only has 15-minute steps, then adjusts by what the stems show. Long, thin stems with small caps point to too much CO2 or too little light.

    What does a Martha tent cost to run?

    About $5 a month in electricity: a 24 W humidifier misting half the day, a 20 W inline fan running around the clock and a controller use about 0.85 kWh a day. Water can cost more than power if you buy distilled water by the gallon.

    Where does Hypha come in?

    Hypha tracks every block in the tent from inoculation to harvest, with tasks and reminders for each grow and a photo journal. Its own sensor, Hypha Aeris (CO₂, temperature and humidity), isn't for sale yet; the app works without it.

    Get Hypha free

    Free forever · 60 days of Pro included · No credit card · iOS, Android & web

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