Bradford protein calculator
Turn a Bradford reading into protein concentration: absorbance 0.42 on a curve with slope 0.035 and intercept 0.05 gives just over ten and a half, in the units of your standards. Diluted wells are scaled back up. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Absorbance
- 0.42
- Slope
- 0.035
- Intercept
- 0.05
- Dilution
- 1
- Concentration
- 10.5714285714
The Bradford protein assay remains one of the most common colorimetric methods for quantifying total protein in purified samples, lysates, and process intermediates. After dye binding, a spectrophotometer reports absorbance—typically near five hundred ninety-five nanometers—and labs convert that reading into concentration using a standard curve built from known protein standards such as BSA. This Bradford protein concentration calculator accepts a blank-corrected sample absorbance together with the slope and intercept of a fitted linear standard curve A equals m times C plus b, optionally multiplies by a fold dilution when the measured well was diluted, and returns the stock concentration with a stable rounded numeric contract.
How Bradford absorbance becomes protein concentration
Coomassie Brilliant Blue G-250 shifts its absorbance spectrum when it binds protein under acidic assay conditions, so wells with more protein appear more blue and read a higher absorbance at the assay wavelength. Absolute absorbance alone is not a universal concentration: dye lot, temperature, path length, protein composition, and instrument blank all move the response.
Laboratories therefore prepare a series of standards at known concentrations, measure their absorbances under the same protocol as the unknowns, and fit a linear model of the form absorbance equals slope times concentration plus intercept over the linear working range. Once slope m and intercept b are known, any sample absorbance A inverts to concentration C equals (A minus b) divided by m.
If the sample was diluted before the assay—for example five-fold so that the reading stays inside the standard curve—the undiluted stock concentration is that inverted value multiplied by the fold dilution. This capability does not fit the standard curve for you, does not average replicate wells, and does not apply non-linear four-parameter logistic models; it applies the linear inversion you already trust from spreadsheet or plate-reader software. Zero slope is rejected because division by zero cannot recover concentration.
Slightly negative computed concentrations can appear when a noisy sample sits below the intercept; the arithmetic still returns the number so quality-control scripts can flag out-of-range wells instead of hiding them behind a silent clamp to zero.
Lab workflows, limits, and search intent for this tool
Typical workflow: run BSA or other protein standards, fit the linear region to obtain slope and intercept, blank-correct the sample absorbance, enter those three numbers, set dilution when the measured aliquot was diluted, and log concentration next to the sample identifier. Protein purification fractions, cell-lysate equal-loading checks, ELISA sample prep, and teaching labs all need the same deterministic inversion without reopening a spreadsheet.
Limits: this tool does not replace the need for standards, does not correct for non-linear dye response at high protein load, does not convert between BSA-equivalent and true analyte mass for every protein species, and does not choose units—results are in whatever concentration units the standard curve used (commonly micrograms per milliliter or milligrams per milliliter). Dilute samples that read above the highest standard into the linear range and pass the fold dilution rather than extrapolating blindly.
Search intent for this landing includes Bradford protein concentration calculator, Bradford assay absorbance to concentration, protein standard curve slope intercept online, and A595 protein quantification from linear regression.
What you can do with it
Purification fractions and load checks
Convert each Bradford A595 reading with your plate’s slope and intercept into concentration for column fractions, equal protein loading, and process intermediate release.
LIMS and plate-reader automation
Pipe spectrophotometer or microplate absorbance values into a fixed linear standard-curve inversion stored per kit lot, with fold dilution when wells were pre-diluted.
Teaching Bradford standard curves
Show students how A = m·C + b inverts to C = (A − b)/m, and how a 1:5 dilution multiplies the undiluted stock concentration by five.
FAQ
How do I calculate protein concentration from a Bradford assay?
Send non-negative absorbance plus the linear standard-curve slope (non-zero) and intercept.
What are slope and intercept?
They come from fitting your protein standards: absorbance ≈ slope × concentration + intercept over the linear range. Use the same blank, wavelength, path length, and units for samples and standards.
When do I set the dilution field?
When the absorbance was measured on a diluted aliquot. For a 1:5 dilution set dilution to 5 so the result estimates the undiluted stock. Leave it at 1 for undiluted samples.
Why is a negative absorbance rejected?
A negative blank-corrected reading usually means a bad blank or the wrong wavelength, not a negative amount of protein. The calculator refuses it instead of silently flipping the sign, so the error is caught at the bench.
What happens if the slope is zero?
It is refused, because the concentration is found by dividing by the slope, and a flat standard curve cannot turn absorbance into an amount. Refit the standards before trying again.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/bio/bradford-protein \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"absorbance":0.42,"slope":0.035,"intercept":0.05}'const res = await fetch("https://api.kit.forhosting.com/bio/bradford-protein", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"absorbance": 0.42,
"slope": 0.035,
"intercept": 0.05
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/bio/bradford-protein",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"absorbance": 0.42,
"slope": 0.035,
"intercept": 0.05
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/bio/bradford-protein", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"absorbance":0.42,"slope":0.035,"intercept":0.05}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"absorbance":0.42,"slope":0.035,"intercept":0.05}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/bio/bradford-protein", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"absorbance": 0.42,
"slope": 0.035,
"intercept": 0.05
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "bio.bradford_protein",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Zero slope, negative absorbance, and non-positive dilution fail with invalid_input instead of inventing a titer.
How Bradford absorbance becomes protein concentration
Blank-corrected absorbance should already be non-negative before you call the API: a negative reading usually means a bad blank or wrong wavelength, not a negative protein mass, so negative absorbance is rejected rather than absolute-valued silently.
Input contract, response fields, and validation rules
Example: absorbance zero point forty-two, slope zero point zero three five, intercept zero point zero five yields concentration ten point five seven one four two eight five seven one four two nine in the standard-curve units; the same absorbance on a five-fold diluted aliquot with dilution five multiplies that inverted value by five for the undiluted stock. Keep slope and intercept from the same fit, blank, wavelength, and path length as the sample well; mixing curves across instruments without renormalizing will mis-scale concentration even when the arithmetic is correct. POST a JSON body with three required finite numbers and one optional factor. Required absorbance is the blank-corrected sample reading—aliases include od, a595, sample_absorbance, abs, and A—and must be non-negative. Required slope is the non-zero linear coefficient m from A equals m times C plus b, with aliases m, standard_slope, curve_slope, and calibration_slope; units are absorbance per concentration unit and must match how intercept and standards were fitted. Required intercept is the finite y-intercept b, with aliases b, standard_intercept, curve_intercept, and y_intercept; it may be positive, negative, or zero. Optional dilution defaults to one for undiluted measurements; when present it must be a finite positive fold dilution (aliases dilution_factor, df, fold_dilution, dil). On success the response echoes absorbance, slope, intercept, and dilution with stable twelve-decimal rounding, plus concentration as the primary numeric result, formula restating whether dilution was unity, and message as a short English summary. Missing fields, non-finite strings, boolean types, negative absorbance, zero slope, and non-positive dilution fail with invalid_input and are not billed. Pricing for successful work is a flat $0.002 with unit item.
Lab workflows, limits, and search intent for this tool
Automation scripts can store slope and intercept per plate or per kit lot and call the API after each absorbance capture. Failed validation does not consume prepaid balance, so exploratory scripts that probe empty slopes stay cheap. Round-trip checks are straightforward: recompute (absorbance minus intercept) divided by slope times dilution and confirm equality with concentration within published rounding, then confirm that zero slope or negative absorbance flips the response from success to invalid_input with a clear field message.
Technical questions
Why was my request rejected?
Negative absorbance, zero slope, non-positive dilution, missing fields, or non-finite values fail with invalid_input and are not billed.
What does the Bradford protein concentration calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.