Macronutrient Dosing Calculator

Enter your tank volume and desired NO3 / PO4 increases to calculate precise KNO3 and KH2PO4 doses in grams and teaspoons.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Tank Volume (gal)
Input the total water volume of your aquarium in US gallons. Accurate volume is crucial for correct dosing.
2
Enter Raise NO3 by (ppm)
Specify how many parts per million (ppm) of nitrate (NO3) you wish to add to your tank. Typical targets for planted tanks are 5-15 ppm.
3
Enter Raise PO4 by (ppm)
Specify how many ppm of phosphate (PO4) you wish to add. Common targets are 0.5-2 ppm, often targeting a 10:1 NO3:PO4 ratio.
4
Review Your Doses
The calculator will provide precise doses in grams and teaspoons for both KNO3 (potassium nitrate) and KH2PO4 (monopotassium phosphate), along with your target NO3:PO4 ratio.

Example Calculation

An aquarist with a 55-gallon planted tank needs to boost nutrient levels to support plant growth after noticing deficiencies.

Tank Volume (gal)

55 gal

Raise NO3 by (ppm)

10 ppm

Raise PO4 by (ppm)

1 ppm

Results

3.47 g

Tips

Measure Accurately with a Scale

For doses under 1 gram, household measuring spoons can be imprecise. Invest in a digital scale accurate to 0.01 grams for consistent and safe dosing, especially for sensitive aquatic environments.

Dose Gradually and Observe

Instead of a single large dose, consider splitting your calculated dose into smaller, more frequent applications (e.g., 2-3 times per week). This helps prevent sudden nutrient spikes and allows you to observe plant response.

Test Water Parameters Regularly

Always verify your water parameters (NO3, PO4, pH, KH, GH) before and after dosing. This confirms the calculator's effectiveness for your specific tank and helps identify any unexpected nutrient interactions or consumption rates.

Precision Macronutrient Dosing for Planted Aquariums

The Macronutrient Dosing Calculator provides precise measurements for adding essential nutrients like nitrate (NO3) and phosphate (PO4) to your planted aquarium. This tool is vital for aquarists practicing methods like the Estimative Index (EI) or Perpetually Dosing System, ensuring aquatic plants receive optimal nutrition for lush growth while minimizing algae. Achieving the right balance, often targeting a 10:1 NO3:PO4 ratio, is critical for a thriving underwater ecosystem in 2025, preventing deficiencies that can lead to stunted plants and algae outbreaks.

The Calculations Behind Aquarium Nutrient Levels

This calculator determines the required grams and teaspoons of potassium nitrate (KNO3) and monopotassium phosphate (KH2PO4) to achieve your target NO3 and PO4 levels. It first converts your tank's volume from US gallons to liters, then applies specific conversion factors based on the molecular weight and nutrient content of each salt. For example, roughly 1 gram of KNO3 per liter raises NO3 by ~0.6 ppm, while 1 gram of KH2PO4 per liter raises PO4 by ~0.7 ppm.

KNO3 (g) = (Target NO3 ppm × Liters) / 600
KH2PO4 (g) = (Target PO4 ppm × Liters) / 700

Where Liters is Tank Volume (gallons) × 3.78541. The denominators (600 and 700) are derived from conversion factors (0.6 and 0.7 ppm per mg/L) adjusted for grams and liters.

💡 Just as this tool helps scale nutrient doses, our Pie Crust Quantity Calculator assists in scaling baking ingredients for different pan sizes.

Dosing Nutrients for a 55-Gallon Planted Tank

Consider an aquarist managing a 55-gallon tank who wants to raise nitrate (NO3) by 10 ppm and phosphate (PO4) by 1 ppm to support plant growth.

Convert Gallons to Liters: 55 gallons × 3.78541 liters/gallon = 208.19755 liters.
Calculate KNO3 Dose: (10 ppm NO3 × 208.19755 liters) / 600 = 3.47 grams of KNO3.
Calculate KH2PO4 Dose: (1 ppm PO4 × 208.19755 liters) / 700 = 0.297 grams of KH2PO4.

Therefore, the aquarist would add 3.47 grams of KNO3 and 0.297 grams of KH2PO4 to achieve their target nutrient levels, resulting in an ideal 10:1 NO3:PO4 ratio.

💡 To ensure your nutrient ratios remain consistent across different batch sizes, our Pie Filling Quantity Scaler offers a similar approach for maintaining proportions.

Balancing Aquatic Plant Nutrition

Maintaining optimal macronutrient levels is paramount for a vibrant planted aquarium. Nitrogen (as nitrate) is critical for leaf and stem growth, while phosphorus (as phosphate) is essential for root development, flowering, and overall plant energy transfer. When these nutrients are out of balance, plants can show signs of deficiency, such as yellowing leaves (nitrogen) or stunted growth (phosphorus), and become more susceptible to algae overgrowth. Many experienced aquarists aim for a nitrate level between 5-20 ppm and phosphate between 0.5-2 ppm, with a preferred NO3:PO4 ratio around 10:1 to foster robust plant health and minimize the competitive advantage for common algae species like GSA (Green Spot Algae).

Standard Nutrient Targets for Planted Aquariums

For most high-tech planted aquariums, maintaining a consistent nutrient supply is key to preventing deficiencies and algae. Industry benchmarks suggest target ranges for various macronutrients and micronutrients. Nitrate (NO3) levels are typically kept between 5-20 ppm, while phosphate (PO4) is often targeted at 0.5-2 ppm, aiming for an NO3:PO4 ratio of roughly 10:1. Potassium (K) is another crucial macronutrient, usually maintained at 10-30 ppm. For micronutrients, iron (Fe) is commonly dosed to achieve 0.1-0.5 ppm, and magnesium (Mg) at 5-10 ppm. These ranges are not absolute rules but provide a robust framework for aquarists to adapt based on plant biomass, light intensity, CO2 injection, and specific plant species' needs, ensuring a thriving aquatic environment.

Frequently Asked Questions

Why is it important to dose macronutrients in a planted aquarium?

Dosing macronutrients like nitrate (NO3) and phosphate (PO4) is crucial for the health and vigorous growth of aquatic plants in an aquarium. While fish waste provides some nutrients, it's often insufficient for heavily planted tanks, especially with inert substrates. Supplementing ensures plants receive the necessary building blocks, preventing deficiencies that can lead to stunted growth, yellowing leaves, and increased susceptibility to algae outbreaks, maintaining a vibrant ecosystem.

What is the Redfield Ratio and how does it apply to aquariums?

The Redfield Ratio (approximately 16:1 nitrogen to phosphorus by atoms, or roughly 10:1 NO3 to PO4 by weight) describes the optimal nutrient balance for phytoplankton growth in marine environments. While not a strict rule for freshwater planted aquariums, many aquarists use a target NO3:PO4 ratio of around 8:1 to 12:1 to encourage healthy plant growth and help suppress certain types of algae. Maintaining this balance is a common strategy in high-tech planted tanks.

Can over-dosing macronutrients harm an aquarium?

Yes, over-dosing macronutrients can certainly harm an aquarium. Excess nitrates and phosphates can fuel severe algae blooms, making the tank unsightly and potentially outcompeting desirable plants for light and resources. Very high nutrient levels might also stress some sensitive fish or invertebrates, although most common aquarium inhabitants tolerate a wide range of nitrate levels. Always aim for target ranges and monitor your tank's response carefully.

What are KNO3 and KH2PO4 and why are they used for dosing?

KNO3 (potassium nitrate) and KH2PO4 (monopotassium phosphate) are common, readily available, and safe chemical salts used by aquarists to add nitrogen (as nitrate) and phosphorus (as phosphate) to planted aquariums. These specific compounds are chosen because they dissolve easily in water, are highly bioavailable to plants, and also provide potassium (K), another essential macronutrient, without significantly altering other water parameters like pH or hardness when dosed appropriately.