In addition to the excellent answer of @J.C.Tarafdar, I would like to add a simple equation (I feel you like so), assuming the moisture content is per meter per unit area.

then if you have a ratio of moisture in m3/m3 Then multiply by 1000 to achieve the value in mm as you requested, which is a short unit of the following "mm water per meter depth of soil per unit area". 

If you are dealing with root depth, say of 35 cm, (350 mm), then convert the m3/m3  by multiplying by the 350 instead of 1000, thus the mm unit is equivalent to  "mm water per 35 cm depth of soil per unit area". 

hope this helps.

The soil moisture calculations in percentages on a weight basis have been commonly used, but this does not give a true pictures of soil-moisture relationships. Two soils may have similar moisture content on a weight basis but not a volume basis. Calculations on a volume basis are more meaningful and practical because water is retained in the soil within a given volume and plant roots also absorb moisture from a volume of soil. % moisture by volume = % moisture in soil by weight x bulk density of the soil

When calculated say for a depth of 12 inches, this volume indicates water in inches per foot depth. On a hectare basis, this value written as hectare inches per hectare foot of soil.

In general, m3/m3 can be converted to 1000 mm of water if 1mm of water content is defined as 1mm of water drawn from 1m deep soil.

In addition to the excellent answer of @J.C.Tarafdar, I would like to add a simple equation (I feel you like so), assuming the moisture content is per meter per unit area.

then if you have a ratio of moisture in m3/m3 Then multiply by 1000 to achieve the value in mm as you requested, which is a short unit of the following "mm water per meter depth of soil per unit area". 

If you are dealing with root depth, say of 35 cm, (350 mm), then convert the m3/m3  by multiplying by the 350 instead of 1000, thus the mm unit is equivalent to  "mm water per 35 cm depth of soil per unit area". 

hope this helps.

dear experts

Thank you so much for adding your answer.

Dear Sir: I am using AMSR2 Satellite data , for Soil Moisture. The values are in percentage which is between 0 to 99%. How to convert soil moisture which is volumetric soil moisture given by AMSR2 into mm.

If I multiply by 100 as suggested then it goes to 990mm . But my Can the soil moisture contents be up to this value

Hi,

I want to convert m3/m3 soil moisture ERA-5 data for top 5 cm soil layer to cm or mm. So this means to multiply volumetric soil moisture by 5cm(50mm) ?

The term volumetric water content (VWC) refers to a given volume of water contained in given volume of substrate (soil or any other soiless media that can soak water). this makes the term have a unit of measurement m3/m3. that means a given m3 of water contained in a given m3 of soil or other media. In short the units (m3/m3) cancels out virtually. So if your device measure for instance 20 % VWC in a soil depth of say 40 cm, it means, of the 40 cm depth in the soil there contains 20 % water. Hence the water content in terms of depth (cm),i.e. 40 cm soil depth is 20% of 40 cm=20/100 * 40 cm= 8 cm. if you wanted to have your final answer in mm then you convert 8 cm to mm which you multiply by 10 as the conversion factor from cm to mm. Your final answer therefore = 80 mm. this 80 mm you have as your final answer means, when you dig 40 cm deep in the earth of soil you will have 80 mm of water in that soil. I hope it is clear.