Curve-fitting in Excel or Libreoffice and API service

Excel add-in for curve fitting

Assayfit Pro is an Excel add-in and API service for Mac Os and Windows. Curve fitting can be performed directly from measured data in Excel or from virtually any other application. Download Add-in

New Libreoffice and Openoffice templates

Assayfit pro now supports curve fitting from Libreoffice and openoffice.
Templates are available in the example section

View examples

Improved online template

Assayfit pro can be used directly from a web page.
Try our latest online template here by clicking on this button

fit online

Mac Os, Windows and Linux, Excel, Libreoffice and others

Any software and cross platform

Assayfit Pro curve fitting for laboratory assays and other scientific data provides maximum flexibility as it can be used on any operating system and from many existing software packages.
Use curve fitting functions like four parameter logistic, five parameter logistic and linear and Passing Bablok regression in Excel, Python, R and online. Create calibration curves and calculate unknown values.
Use it to automate curve fitting and write your own scripts.
A free API key is available. Paid plans provide additional features. Join now and try examples in Excel, R, Python and others.

How to use

Free version of Assayfit Pro

To use the free version obtain a free API key at, no credit card required, and download the templates for Excel, R, Python, Java and C sharp for Assayfit Pro.
You can use the API key for two weeks with unlimited renewal options after your key is expired.
The free API key will allow you to use 50 curve fits with almost identical features as the payed versions

Paid plans

Paid plans provide an unlimited number of curve-fits and more data points (up to 500). You can choose from three paid plans, the INDIVIDUAL, GROUP and PREMIUM plans.
The INDIVIDUAL plan is a single user plan, the GROUP key is a multi-user plan.
The PREMIUM plan is a custom plan for large data sets.
Compare plans or view the full feature list below. Additional information is available on

Excel curve fitting


Fit functions

  • Linear curve fitting
  • Linear Passing-Bablok regression
  • 4PL Four parameter logistic
  • 5PL Five parameter logistic
  • second order polynomial
  • third order polynomial
  • Point to point (cubic spline)
  • Univariate spline
  • your own fit function (inquire)

Fit input parameters

  • x and y data
  • Weights
  • Custom start values
  • Apply fit limits
  • fit function
  • Extrapolate
  • Calculate unknown y value from calibration curve
  • Calculate unknown x value from calibration curve
  • 96 well input
  • Plate layout
  • Sample ID

Output parameters

  • xdata (your standard x values)
  • ydata (your standard y values)
  • residuals
  • weights
  • parameter start values
  • parameters after optimisation
  • SD or upper an lower limit of the parameters (for some functions)
  • weighted sum of squares
  • correlation coefficient (for linear fits)
  • Calibration curve limits
  • yknown values (values of which you known the y value and want to calculate the fitted x value)
  • x values calculated from y values
  • xknown values (values of which you know the x value and want to calculate the fitted y value
  • y calculated from xknown values
  • x coordinates of the fit curve (100)
  • y coordinates of the fit curve (100)


  • Javascript curve fitting
  • Standard curve Excel
  • Dose response curve excel
  • Polynomial regression Excel
  • Excel automation example
  • Nonlinear regression in r
  • Curve fit python
  • C# curve fitting
  • template
  • (linear) regression java
  • Any other sofware which can send http get and post commands

Cross platform use

  • Excel Add-in for Mac OS and Windows
  • Libreoffice - Openoffice templates fro Mac Os, Windows and Linux.
  • Software API to create your own software
  • Online curve fitting tool


  • Support over email and phone (only if required)
  • Help with creating templates or software (inquire)
  • Free trial of full service


Choose a payment plan that works for you


50 curve fits per period

2 weeks valid

20 standards

96 unknown values

Registration required


Unlimited curve fits

1 month up to one year

500 standards

500 unknown values

Individual account


Unlimited curve fits

1 month up to one year

500 standards

500 unknown values

Share with your team


Unlimited curve fits

1 month up to one year

5000 standards

5000 unknown values

Share with your team

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