BOD Calculator

Last updated: April 5, 2026

The BOD Calculator computes Biochemical Oxygen Demand (BOD₅) from initial and final dissolved oxygen readings using the standard APHA dilution method. BOD₅ is the primary indicator of organic pollution in water and wastewater — it quantifies the oxygen consumed by microorganisms over 5 days at 20°C.

Calculator

Initial Dissolved Oxygen (DO₁)mg/L

Final Dissolved Oxygen (DO₂)mg/L

Sample Volume in BOD BottlemL

BOD Bottle VolumemL

Seed Correctionmg/L

Results

BOD

26.5

mg/L

DO Consumed

5.3

mg/L

Dilution Factor

Sample Fraction

Final DO Remaining

37.65

% saturation basis

Results

BOD

26.5

mg/L

DO Consumed

5.3

mg/L

Dilution Factor

Sample Fraction

Final DO Remaining

37.65

% saturation basis

When organic pollution enters a waterway, bacteria consume it by using dissolved oxygen. If the organic load is heavy enough, bacteria deplete oxygen faster than it replenishes, killing fish and aquatic invertebrates. BOD₅ measures this oxygen demand quantitatively — it is the benchmark regulators, engineers, and environmental scientists use to assess organic pollution, design treatment systems, and set discharge permits. The BOD calculator computes BOD₅ from your laboratory dissolved oxygen measurements using the standard APHA method.

BOD₅ Calculation Formula

BOD₅ (mg/L) = (DO_initial − DO_final) ÷ P

where P = decimal dilution fraction (sample volume ÷ total bottle volume). For a 6 mL sample in a 300 mL bottle: P = 6/300 = 0.02. If DO_initial = 8.0 mg/L and DO_final = 5.5 mg/L after 5 days: BOD₅ = (8.0 − 5.5) ÷ 0.02 = 125 mg/L. For seeded samples (where dilution water is inoculated with bacteria), subtract the seed blank oxygen depletion: BOD₅ = [(DO_initial − DO_final) − (Blank_initial − Blank_final) × f] ÷ P, where f = seed fraction correction. Use this online calculator for your lab measurements. The biochemical oxygen demand calculator provides BOD estimation from kinetic rate constants.

BOD₅ Reference Values for Common Water Types

Pristine river water: below 2 mg/L — healthy, supports sensitive species
Slightly enriched: 2–5 mg/L — some organic input; moderate aquatic life impact
Moderately polluted: 5–10 mg/L — significant DO depletion risk in warm weather
Heavily polluted: 10–20 mg/L — most fish species affected
Raw municipal sewage: 100–300 mg/L
Secondary treated effluent (WWTP): 10–30 mg/L
Food/dairy processing wastewater: 500–5,000+ mg/L

US EPA secondary treatment effluent standard: BOD₅ below 30 mg/L as a 30-day average.

Why Samples Must Be Diluted Before Testing

Most wastewater samples are too concentrated to test undiluted — they would consume all dissolved oxygen within hours, producing a bottomed-out result rather than a true BOD measurement. Dilute so that 20–70% of initial dissolved oxygen is consumed over 5 days (final DO ≥ 2 mg/L; at least 1 mg/L depletion). Typical dilutions: clean surface water — no dilution to 1:5; treated effluent — 1:5 to 1:50; raw sewage — 1:50 to 1:500; industrial food waste — 1:100 to 1:2,000. Run 2–3 dilutions of each sample in parallel to ensure at least one valid result.

BOD₅ vs. COD: Which to Use When

BOD₅: 5-day test; measures biodegradable organic fraction; regulatory standard for effluent permits; slow (5 days) but biologically meaningful
COD: 2–3 hour test; measures all oxidizable matter (biodegradable + non-biodegradable); faster for process control; COD/BOD ratio above 2.5 indicates significant non-biodegradable or toxic fraction

The BOD kinetics calculator and environmental science calculators complete the water quality analysis toolkit.

Visual Analysis

How It Works

Enter initial dissolved oxygen (mg/L), final dissolved oxygen after 5 days at 20°C (mg/L), and the dilution fraction (sample volume ÷ total bottle volume). BOD₅ = (DO_initial − DO_final) ÷ P. For seeded dilution: BOD₅ = [(DO_initial − DO_final) − (Blank_i − Blank_f) × f] ÷ P. Results in mg/L of the undiluted sample.

Worked Examples

Municipal Wastewater

Inputs

do initial8.5

do final3.2

dilution factor5

Results

bod26.5

do consumed5.3

BOD = (8.5 - 3.2) x 5 = 26.5 mg/L. This is typical of partially treated wastewater effluent.

Clean River Water

Inputs

do initial9

do final8.4

dilution factor1

Results

bod0.6

do consumed0.6

BOD of 0.6 mg/L indicates very clean water with minimal organic pollution. No dilution needed.

Frequently Asked Questions

BOD₅ is the amount of dissolved oxygen consumed by microorganisms decomposing organic matter in a water sample over exactly 5 days at 20°C (68°F). It is expressed in mg/L and is the standard indicator of organic pollution in water bodies and wastewater. High BOD means high organic content — when high-BOD water is discharged to a river, bacteria consume the organic matter while depleting dissolved oxygen, potentially creating hypoxic zones that kill fish and aquatic invertebrates. BOD₅ is the primary parameter in US EPA effluent discharge permits for wastewater treatment plants, with secondary treatment standards requiring BOD₅ ≤30 mg/L as a 30-day average.

The standard BOD₅ test (APHA Method 5210B): collect a representative water sample; dilute with oxygen-saturated dilution water to the appropriate concentration; measure initial dissolved oxygen with a calibrated DO meter; seal bottles without headspace; incubate in the dark at 20°C ±1°C for exactly 5 days; measure final DO; calculate BOD₅ = (DO_i − DO_f) ÷ dilution fraction. Valid results require: DO depletion of at least 2 mg/L (insufficient depletion if too dilute); final DO of at least 1 mg/L remaining (too concentrated if final DO is zero). Run blank dilution water controls in parallel. For samples with low microbial populations (some industrial wastes, treated effluent), add a seed (dilute domestic wastewater) to provide active bacteria.

US EPA effluent guidelines for secondary wastewater treatment: BOD₅ ≤30 mg/L as a 30-day average and ≤45 mg/L as a 7-day average. Advanced secondary treatment achieves BOD₅ of 10–20 mg/L. Tertiary treatment (additional filtration, biological nutrient removal) achieves BOD₅ below 5 mg/L. For receiving water quality: most aquatic life guidelines require BOD₅ ≤5 mg/L in the receiving water body after dilution and mixing. A well-operated activated sludge plant treating typical municipal sewage (influent BOD₅ 150–250 mg/L) routinely achieves 90–95% removal efficiency, producing effluent at 10–25 mg/L BOD₅.

BOD (Biochemical Oxygen Demand) measures only biodegradable organic matter — oxygen consumed by bacteria in 5 days. COD (Chemical Oxygen Demand) measures all oxidizable matter using a chemical oxidant, including non-biodegradable organics, in 2–3 hours. BOD is the regulatory standard because it reflects the actual biological oxygen depletion impact on water bodies. COD is faster and used for daily process control at wastewater treatment plants. The COD/BOD ratio characterizes biodegradability: ratio close to 1 = mostly biodegradable (amenable to biological treatment); ratio above 2.5–3 = significant non-biodegradable or toxic fraction (may need pre-treatment). For typical domestic sewage, COD/BOD ≈ 1.5–2.0. For industrial food waste, COD/BOD ≈ 1.2–1.5 (highly biodegradable).

The 20°C (68°F) incubation temperature is standardized to ensure reproducibility of BOD measurements worldwide, independent of ambient temperature. It represents a compromise between typical river water temperatures in temperate climates and laboratory room temperature. Microbial activity (and therefore BOD) increases with temperature — the same sample would show higher oxygen depletion at 30°C than at 20°C. The BOD temperature correction factor (θ = 1.047) allows conversion between temperatures: BOD_T = BOD_20 × 1.047^(T-20). This means BOD in a summer river at 25°C is approximately 27% higher than the same water would show in the lab test at 20°C. All regulatory BOD measurements are reported at the standard 20°C condition.

Seeded BOD testing adds a small amount of microbially active water (the 'seed') to the dilution water when the sample itself lacks sufficient microorganisms to produce a measurable oxygen depletion. Seeding is needed for: highly chlorinated effluents (chlorine kills bacteria, so adding seed restores microbial activity); industrial effluents with no indigenous microbial population; very clean water samples where indigenous bacteria are too few for reliable decomposition in 5 days. Common seed sources: domestic wastewater (settled), effluent from a wastewater treatment plant, or commercial BOD seed preparations. When seeding, run paired blank bottles (dilution water + seed, no sample) to measure the seed's oxygen demand alone, then subtract that contribution from the sample result — this is the seed correction in the standard BOD formula.

Sources & Methodology

APHA, AWWA, WEF (2017). Standard Methods for the Examination of Water and Wastewater, 23rd ed. Method 5210B. US EPA (2016). Method OW 5210B. Metcalf & Eddy / AECOM (2013). Wastewater Engineering, 5th ed.

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How It Works

Worked Examples

Municipal Wastewater

Inputs

do initial8.5

do final3.2

dilution factor5

Results

bod26.5

do consumed5.3

BOD = (8.5 - 3.2) x 5 = 26.5 mg/L. This is typical of partially treated wastewater effluent.

Clean River Water

Inputs

do initial9

do final8.4

dilution factor1

Results

bod0.6

do consumed0.6

BOD of 0.6 mg/L indicates very clean water with minimal organic pollution. No dilution needed.

Frequently Asked Questions