diff --git a/CMakeLists_files.cmake b/CMakeLists_files.cmake
index 9fb202e48..2e6a59963 100644
--- a/CMakeLists_files.cmake
+++ b/CMakeLists_files.cmake
@@ -312,6 +312,7 @@ if(ENABLE_ECL_INPUT)
   list(APPEND TEST_SOURCE_FILES
     tests/rst_test.cpp
     tests/test_ERsm.cpp
+    tests/test_GuideRate.cpp
     tests/parser/ACTIONX.cpp
     tests/parser/ADDREGTests.cpp
     tests/parser/AquiferTests.cpp
diff --git a/tests/test_GuideRate.cpp b/tests/test_GuideRate.cpp
new file mode 100644
index 000000000..8e522589a
--- /dev/null
+++ b/tests/test_GuideRate.cpp
@@ -0,0 +1,524 @@
+/*
+  Copyright (c) 2020 Equinor ASA
+
+  This file is part of the Open Porous Media project (OPM).
+
+  OPM is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  OPM is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with OPM.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#define BOOST_TEST_MODULE test_GuideRate
+
+#include <boost/test/unit_test.hpp>
+
+#include <opm/parser/eclipse/EclipseState/Schedule/Group/GuideRate.hpp>
+
+#include <opm/parser/eclipse/Deck/Deck.hpp>
+#include <opm/parser/eclipse/Parser/ErrorGuard.hpp>
+#include <opm/parser/eclipse/Parser/ParseContext.hpp>
+#include <opm/parser/eclipse/Parser/Parser.hpp>
+#include <opm/parser/eclipse/Python/Python.hpp>
+
+#include <opm/parser/eclipse/EclipseState/EclipseState.hpp>
+#include <opm/parser/eclipse/EclipseState/Grid/EclipseGrid.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/Group/Group.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/Well/Well.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/Schedule.hpp>
+
+#include <opm/parser/eclipse/Units/Units.hpp>
+
+#include <memory>
+#include <string>
+#include <utility>
+
+#include <stddef.h>
+
+namespace {
+    struct Setup
+    {
+        explicit Setup(const std::string& input)
+            : Setup { Opm::Parser{}.parseString(input) }
+        {}
+
+        explicit Setup(const Opm::Deck& deck)
+            : es    { deck }
+            , sched { deck, es, std::make_shared<const Opm::Python>() }
+            , gr    { sched }
+        {}
+
+        Opm::EclipseState es;
+        Opm::Schedule     sched;
+        Opm::GuideRate    gr;
+    };
+
+    Setup case_10x10x10_model4(const double damping_factor = 0.5)
+    {
+        const auto prolog = std::string { R"(RUNSPEC
+START
+  4 'AUG' 2020 /
+
+TITLE
+  Check GUIDERAT Formula Implementation
+
+DIMENS
+  10 10 10 /
+
+OIL
+GAS
+WATER
+DISGAS
+VAPOIL
+
+METRIC
+
+TABDIMS
+/
+
+WELLDIMS
+3 10 2 2 /
+
+GRID
+
+DXV
+  10*100 /
+
+DYV
+  10*100 /
+
+DZV
+  10*5 /
+
+DEPTHZ
+  121*2000 /
+
+PERMX
+  1000*100 /
+
+COPY
+  PERMX PERMY /
+  PERMX PERMZ /
+/
+
+MULTIPLY
+  PERMZ 0.1 /
+/
+
+PORO
+  1000*0.3 /
+
+SOLUTION
+
+PRESSURE
+  1000*320 /
+SOIL
+  1000*0.85 /
+SWAT
+  1000*0.12 /
+SGAS
+  1000*0.03 /
+RS
+  1000*226.0 /
+RV
+  1000*0.0 /
+
+SCHEDULE
+
+WELSPECS
+  P1 P 10 7 2002.5 OIL /
+  P2 P 7 10 2002.5 OIL /
+  I1 I 2  2 2002.5 GAS /
+/
+
+COMPDAT
+  P1 2* 1 10 OPEN 1* 1* 0.5 /
+  P2 2* 1 10 OPEN 1* 1* 0.5 /
+  I1 2* 1 10 OPEN 1* 1* 0.5 /
+/
+)"  };
+
+        const auto guiderat = std::string { R"(
+-- GR_{oil} = WOPP / (0.5 + (WWPP / WOPP))
+-- with a user-specified damping/time-delay factor (default 0.5).
+GUIDERAT
+--1   2   3   4   5   6   7  8  9          10
+  1.0 OIL 1.0 0.5 1.0 1.0 1* 1* YES )" } + std::to_string(damping_factor) + " /\n";
+
+        const auto epilog = std::string { R"(
+WCONPROD
+  P* OPEN GRUP 150 100 15E+3 250 1* 50 25 /
+/
+
+WCONINJE
+  I1 GAS OPEN RATE 20.0E+3 1* 500 350 /
+/
+
+GCONPROD
+  P 'ORAT' 200.0 150.0 100.0E+3 1* 1* YES 1* FORM /
+/
+
+DATES
+  5 'AUG' 2020 /
+ 10 'AUG' 2020 /
+ 20 'AUG' 2020 /
+  1 'SEP' 2020 /
+  1 'OCT' 2020 /
+  1 'NOV' 2020 /
+  1 'DEC' 2020 /
+  1 'JAN' 2021 /
+/
+
+END
+)" };
+
+        return Setup { prolog + guiderat + epilog };
+}
+} // Namespace anonymous
+
+// ======================================================================
+
+BOOST_AUTO_TEST_SUITE(GuideRate_Calculations)
+
+BOOST_AUTO_TEST_CASE(P1_First)
+{
+    auto cse = case_10x10x10_model4();
+
+    const auto wopp = 1.0;
+    const auto wgpp = 5.0;
+    const auto wwpp = 0.1;
+    const auto stm  = 0.0;
+    const auto rpt  = size_t{1};
+
+    cse.gr.compute("P1", rpt, stm, wopp, wgpp, wwpp);
+
+    const auto orat = 2.0;
+    const auto grat = 4.0;      // == 2 * orat
+    const auto wrat = 1.0;      // == orat / 2
+
+    const auto expect_gr_oil = 1.0 / (0.5 + 0.1/1.0); // wopp / (0.5 + wwpp/wopp)
+
+    // GR_{oil}
+    {
+        const auto grval = cse.gr.get("P1", Opm::Well::GuideRateTarget::OIL, { orat, grat, wrat });
+
+        BOOST_CHECK_CLOSE(grval, expect_gr_oil, 1.0e-5);
+    }
+
+    // GR_{gas}
+    {
+        const auto grval = cse.gr.get("P1", Opm::Well::GuideRateTarget::GAS, { orat, grat, wrat });
+
+        const auto expect = (grat / orat) * expect_gr_oil;
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{water}
+    {
+        const auto grval = cse.gr.get("P1", Opm::Well::GuideRateTarget::WAT, { orat, grat, wrat });
+
+        const auto expect = (wrat / orat) * expect_gr_oil;
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+}
+
+BOOST_AUTO_TEST_CASE(P2_Second)
+{
+    auto cse = case_10x10x10_model4();
+
+    {
+        const auto wopp = 1.0;
+        const auto wgpp = 5.0;
+        const auto wwpp = 0.1;
+        const auto stm  = 0.0;
+        const auto rpt  = size_t{1};
+
+        cse.gr.compute("P2", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    {
+        const auto wopp = 10.0;
+        const auto wgpp = 50.0;
+        const auto wwpp = 1.0;
+        const auto stm  = 10.0*Opm::unit::second; // Before recalculation delay
+        const auto rpt  = size_t{1};
+
+        cse.gr.compute("P2", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    const auto orat = 2.0;
+    const auto grat = 4.0;      // == 2 * orat
+    const auto wrat = 1.0;      // == orat / 2
+
+    const auto expect_gr_oil_1 = 1.0 / (0.5 + 0.1/1.0); // wopp_1 / (0.5 + wwpp_1/wopp_1)
+
+    // GR_{oil}
+    {
+        const auto grval = cse.gr.get("P2", Opm::Well::GuideRateTarget::OIL, { orat, grat, wrat });
+
+        BOOST_CHECK_CLOSE(grval, expect_gr_oil_1, 1.0e-5);
+    }
+
+    // GR_{gas}
+    {
+        const auto grval = cse.gr.get("P2", Opm::Well::GuideRateTarget::GAS, { orat, grat, wrat });
+
+        const auto expect = (grat / orat) * expect_gr_oil_1;
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{water}
+    {
+        const auto grval = cse.gr.get("P2", Opm::Well::GuideRateTarget::WAT, { orat, grat, wrat });
+
+        const auto expect = (wrat / orat) * expect_gr_oil_1;
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    {
+        const auto wopp = 10.0;
+        const auto wgpp = 50.0;
+        const auto wwpp = 1.0;
+        const auto stm  = 10.0*Opm::unit::day; // After recalculation delay
+        const auto rpt  = size_t{3};
+
+        cse.gr.compute("P2", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    const auto expect_gr_oil_2 = 10.0 / (0.5 + 1.0/10.0); // wopp_2 / (0.5 + wwpp_2/wopp_2)
+
+    // GR_{oil}
+    {
+        const auto grval = cse.gr.get("P2", Opm::Well::GuideRateTarget::OIL, { orat, grat, wrat });
+
+        const auto expect = 0.5*expect_gr_oil_2 + 0.5*expect_gr_oil_1;
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{gas}
+    {
+        const auto grval = cse.gr.get("P2", Opm::Well::GuideRateTarget::GAS, { orat, grat, wrat });
+
+        const auto expect = (grat / orat) * (0.5*expect_gr_oil_2 + 0.5*expect_gr_oil_1);
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{water}
+    {
+        const auto grval = cse.gr.get("P2", Opm::Well::GuideRateTarget::WAT, { orat, grat, wrat });
+
+        const auto expect = (wrat / orat) * (0.5*expect_gr_oil_2 + 0.5*expect_gr_oil_1);
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+}
+
+BOOST_AUTO_TEST_CASE(P_Third)
+{
+    auto cse = case_10x10x10_model4();
+
+    {
+        const auto wopp = 1.0;
+        const auto wgpp = 5.0;
+        const auto wwpp = 0.1;
+        const auto stm  = 0.0;
+        const auto rpt  = size_t{1};
+
+        cse.gr.compute("P", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    {
+        const auto wopp = 10.0;
+        const auto wgpp = 50.0;
+        const auto wwpp = 1.0;
+        const auto stm  = 10.0*Opm::unit::day;
+        const auto rpt  = size_t{3};
+
+        cse.gr.compute("P", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    {
+        const auto wopp = 20.0;
+        const auto wgpp = 100.0;
+        const auto wwpp = 10.0;
+        const auto stm  = 20.0*Opm::unit::day;
+        const auto rpt  = size_t{4};
+
+        cse.gr.compute("P", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    const auto expect_gr_oil_1 =  1.0 / (0.5 +  0.1/ 1.0); // wopp_1 / (0.5 + wwpp_1/wopp_1)
+    const auto expect_gr_oil_2 = 10.0 / (0.5 +  1.0/10.0); // wopp_2 / (0.5 + wwpp_2/wopp_2)
+    const auto expect_gr_oil_3 = 20.0 / (0.5 + 10.0/20.0); // wopp_3 / (0.5 + wwpp_3/wopp_3)
+
+    const auto orat = 2.0;
+    const auto grat = 4.0;      // == 2 * orat
+    const auto wrat = 1.0;      // == orat / 2
+
+    // GR_{oil}
+    {
+        const auto grval = cse.gr.get("P", Opm::Well::GuideRateTarget::OIL, { orat, grat, wrat });
+
+        const auto expect = 0.5*expect_gr_oil_3 + 0.5*0.5*expect_gr_oil_2 + 0.5*0.5*expect_gr_oil_1;
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{gas}
+    {
+        const auto grval = cse.gr.get("P", Opm::Well::GuideRateTarget::GAS, { orat, grat, wrat });
+
+        const auto expect = (grat / orat) * (0.5*expect_gr_oil_3 + 0.5*0.5*expect_gr_oil_2 + 0.5*0.5*expect_gr_oil_1);
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{water}
+    {
+        const auto grval = cse.gr.get("P", Opm::Well::GuideRateTarget::WAT, { orat, grat, wrat });
+
+        const auto expect = (wrat / orat) * (0.5*expect_gr_oil_3 + 0.5*0.5*expect_gr_oil_2 + 0.5*0.5*expect_gr_oil_1);
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+}
+
+BOOST_AUTO_TEST_CASE(P_Third_df01)
+{
+    auto cse = case_10x10x10_model4(0.1);
+
+    {
+        const auto wopp = 1.0;
+        const auto wgpp = 5.0;
+        const auto wwpp = 0.1;
+        const auto stm  = 0.0;
+        const auto rpt  = size_t{1};
+
+        cse.gr.compute("P", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    {
+        const auto wopp = 10.0;
+        const auto wgpp = 50.0;
+        const auto wwpp = 1.0;
+        const auto stm  = 10.0*Opm::unit::day;
+        const auto rpt  = size_t{3};
+
+        cse.gr.compute("P", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    {
+        const auto wopp = 20.0;
+        const auto wgpp = 100.0;
+        const auto wwpp = 10.0;
+        const auto stm  = 20.0*Opm::unit::day;
+        const auto rpt  = size_t{4};
+
+        cse.gr.compute("P", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    const auto expect_gr_oil_1 =  1.0 / (0.5 +  0.1/ 1.0); // wopp_1 / (0.5 + wwpp_1/wopp_1)
+    const auto expect_gr_oil_2 = 10.0 / (0.5 +  1.0/10.0); // wopp_2 / (0.5 + wwpp_2/wopp_2)
+    const auto expect_gr_oil_3 = 20.0 / (0.5 + 10.0/20.0); // wopp_3 / (0.5 + wwpp_3/wopp_3)
+
+    const auto orat = 2.0;
+    const auto grat = 4.0;      // == 2 * orat
+    const auto wrat = 1.0;      // == orat / 2
+
+    // GR_{oil}
+    {
+        const auto grval = cse.gr.get("P", Opm::Well::GuideRateTarget::OIL, { orat, grat, wrat });
+
+        const auto expect = 0.1*expect_gr_oil_3 + 0.1*0.9*expect_gr_oil_2 + 0.9*0.9*expect_gr_oil_1;
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{gas}
+    {
+        const auto grval = cse.gr.get("P", Opm::Well::GuideRateTarget::GAS, { orat, grat, wrat });
+
+        const auto expect = (grat / orat) * (0.1*expect_gr_oil_3 + 0.1*0.9*expect_gr_oil_2 + 0.9*0.9*expect_gr_oil_1);
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{water}
+    {
+        const auto grval = cse.gr.get("P", Opm::Well::GuideRateTarget::WAT, { orat, grat, wrat });
+
+        const auto expect = (wrat / orat) * (0.1*expect_gr_oil_3 + 0.1*0.9*expect_gr_oil_2 + 0.9*0.9*expect_gr_oil_1);
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+}
+
+BOOST_AUTO_TEST_CASE(P_Third_df09)
+{
+    auto cse = case_10x10x10_model4(0.9);
+
+    {
+        const auto wopp = 1.0;
+        const auto wgpp = 5.0;
+        const auto wwpp = 0.1;
+        const auto stm  = 0.0;
+        const auto rpt  = size_t{1};
+
+        cse.gr.compute("P", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    {
+        const auto wopp = 10.0;
+        const auto wgpp = 50.0;
+        const auto wwpp = 1.0;
+        const auto stm  = 10.0*Opm::unit::day;
+        const auto rpt  = size_t{3};
+
+        cse.gr.compute("P", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    {
+        const auto wopp = 20.0;
+        const auto wgpp = 100.0;
+        const auto wwpp = 10.0;
+        const auto stm  = 20.0*Opm::unit::day;
+        const auto rpt  = size_t{4};
+
+        cse.gr.compute("P", rpt, stm, wopp, wgpp, wwpp);
+    }
+
+    const auto expect_gr_oil_1 =  1.0 / (0.5 +  0.1/ 1.0); // wopp_1 / (0.5 + wwpp_1/wopp_1)
+    const auto expect_gr_oil_2 = 10.0 / (0.5 +  1.0/10.0); // wopp_2 / (0.5 + wwpp_2/wopp_2)
+    const auto expect_gr_oil_3 = 20.0 / (0.5 + 10.0/20.0); // wopp_3 / (0.5 + wwpp_3/wopp_3)
+
+    const auto orat = 2.0;
+    const auto grat = 4.0;      // == 2 * orat
+    const auto wrat = 1.0;      // == orat / 2
+
+    // GR_{oil}
+    {
+        const auto grval = cse.gr.get("P", Opm::Well::GuideRateTarget::OIL, { orat, grat, wrat });
+
+        const auto expect = 0.9*expect_gr_oil_3 + 0.9*0.1*expect_gr_oil_2 + 0.1*0.1*expect_gr_oil_1;
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{gas}
+    {
+        const auto grval = cse.gr.get("P", Opm::Well::GuideRateTarget::GAS, { orat, grat, wrat });
+
+        const auto expect = (grat / orat) * (0.9*expect_gr_oil_3 + 0.9*0.1*expect_gr_oil_2 + 0.1*0.1*expect_gr_oil_1);
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+
+    // GR_{water}
+    {
+        const auto grval = cse.gr.get("P", Opm::Well::GuideRateTarget::WAT, { orat, grat, wrat });
+
+        const auto expect = (wrat / orat) * (0.9*expect_gr_oil_3 + 0.9*0.1*expect_gr_oil_2 + 0.1*0.1*expect_gr_oil_1);
+        BOOST_CHECK_CLOSE(grval, expect, 1.0e-5);
+    }
+}
+
+BOOST_AUTO_TEST_SUITE_END() // GuideRate_Calculations